aufräumen

Directory.Packages.props

@@ -19,7 +19,6 @@
 
     <!-- API -->
     <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.0" />
-    <PackageVersion Include="System.Text.Json"                              Version="10.0.0" />
 
     <!-- Tests -->
     <PackageVersion Include="xunit"                  Version="2.9.3" />

LehrerApp.Api/LehrerApp.Api.csproj

@@ -9,6 +9,5 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" />
     <PackageReference Include="LiteDB" />
-    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
 </Project>

LehrerApp.Api/ReadableSnapshot.cs

@@ -7,8 +7,9 @@ public class ReadableSnapshot
     public DateTime ExportedAt { get; set; }
     public ReadableSnapshotMeta Meta { get; set; } = new();
     public List<LearningGroup> Groups { get; set; } = [];
+    public List<Subject> Subjects { get; set; } = [];
     public List<Student> Students { get; set; } = [];
-    public List<Enrollment> Enrollments { get; set; } = [];
+    public List<GroupMembership> Memberships { get; set; } = [];
 }
 
 public class ReadableSnapshotMeta

LehrerApp.Core/Interfaces/IRepositories.cs

@@ -6,7 +6,7 @@ public interface IStudentRepository
 {
     Student? GetById(Guid id);
     List<Student> GetAll(bool includeInactive = false);
-    List<Student> GetByGroup(Guid groupId, string schoolYear);
+    List<Student> GetByGroup(Guid groupId);
     void Save(Student student);
     void Delete(Guid id);
 }
@@ -18,12 +18,12 @@ public interface IGroupRepository
     void Save(LearningGroup group);
     void Delete(Guid id);
 }
-public interface IEnrollmentRepository
+public interface IGroupMembershipRepository
 {
-    List<Enrollment> GetByStudent(Guid studentId);
-    List<Enrollment> GetByGroup(Guid groupId);
-    List<Enrollment> GetByGroupAndYear(Guid groupId, string schoolYear);
-    void Save(Enrollment enrollment);
+    List<GroupMembership> GetByStudent(Guid studentId);
+    List<GroupMembership> GetByGroup(Guid groupId);
+    GroupMembership? GetByStudentAndGroup(Guid studentId, Guid groupId);
+    void Save(GroupMembership membership);
     void Delete(Guid id);
 }
 public interface IExamRepository

LehrerApp.Core/Models/Exam.cs

@@ -6,7 +6,6 @@ public class Exam
     public Guid GroupId { get; set; }
     public string Title { get; set; } = "";
     public DateOnly Date { get; set; }
-    public string Subject { get; set; } = "";
     public int? ExamNumber { get; set; }
     public List<ExamTask> Tasks { get; set; } = [];
     public List<GradingKeyEntry> GradingKey { get; set; } = [];

LehrerApp.Core/Models/LearningGroup.cs

@@ -5,7 +5,6 @@ public class LearningGroup
     public Guid Id { get; set; } = Guid.NewGuid();
     public string Name { get; set; } = "";
     public GroupType Type { get; set; }
-    public string? Subject { get; set; }
     public Guid? SubjectId { get; set; }
     public string SchoolYear { get; set; } = "";
     public int GradeLevel { get; set; }
@@ -17,19 +16,23 @@ public class LearningGroup
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }
-public class Enrollment
+/// <summary>
+/// Mitgliedschaft eines Schülers in einer Lerngruppe. Das Schuljahr gehört zur
+/// Lerngruppe und wird deshalb hier nicht ein zweites Mal gespeichert.
+/// </summary>
+public class GroupMembership
 {
-    public Guid   Id         { get; set; } = Guid.NewGuid();
-    public Guid   StudentId  { get; set; }
-    public Guid   GroupId    { get; set; }
-    public string SchoolYear { get; set; } = "";
-    public DateOnly  EnrolledAt { get; set; } = DateOnly.FromDateTime(DateTime.Today);
-    public EnrollmentPeriod Period   { get; set; } = EnrollmentPeriod.FullYear;
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public Guid StudentId { get; set; }
+    public Guid GroupId { get; set; }
+    /// <summary>Tag, an dem die Zuordnung in der App angelegt wurde.</summary>
+    public DateOnly AddedOn { get; set; } = DateOnly.FromDateTime(DateTime.Today);
+    public MembershipPeriod Period { get; set; } = MembershipPeriod.FullYear;
     public DateOnly? JoinedAt { get; set; }
     public DateOnly? LeftAt   { get; set; }
     public Niveau? Niveau { get; set; }
 }
-public enum EnrollmentPeriod { FullYear, H1Only, H2Only, Custom }
+public enum MembershipPeriod { FullYear, H1Only, H2Only, Custom }
 public enum GroupType { Class, Course }
 public enum GradingSystem { Grades1To6, Points0To15 }
 public enum Niveau { E, G, Foerder }

LehrerApp.Core/Models/Participation.cs

@@ -16,9 +16,7 @@ public class ParticipationEntry
 {
     public Guid Id { get; set; } = Guid.NewGuid();
     public Guid SessionId { get; set; }
-    public Guid GroupId { get; set; }
     public Guid StudentId { get; set; }
-    public DateOnly Date { get; set; } = DateOnly.FromDateTime(DateTime.Today);
     public List<AspectRating>       Ratings           { get; set; } = [];
     public List<CompetencyRating>   CompetencyRatings { get; set; } = [];
     public string? Note { get; set; }

LehrerApp.Core/Models/Planning.cs

@@ -5,7 +5,6 @@ public class Grade
     public Guid Id { get; set; } = Guid.NewGuid();
     public Guid StudentId { get; set; }
     public Guid GroupId { get; set; }
-    public string SchoolYear { get; set; } = "";
     public GradeCategory Category { get; set; }
     public string Value { get; set; } = "";
     public DateOnly Date { get; set; }
@@ -20,8 +19,6 @@ public class Unit
     public Guid Id { get; set; } = Guid.NewGuid();
     public Guid GroupId { get; set; }
     public string Title { get; set; } = "";
-    public string Subject { get; set; } = "";
-    public string SchoolYear { get; set; } = "";
     public DateOnly? StartDate { get; set; }
     public DateOnly? EndDate { get; set; }
     public List<string> Competencies { get; set; } = [];

LehrerApp.Data.Tests/LehrerApp.Data.Tests.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\LehrerApp.Data\LehrerApp.Data.csproj" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.runner.visualstudio">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+</Project>

LehrerApp.Data.Tests/LiteDbContextTests.cs

@@ -0,0 +1,135 @@
+using LehrerApp.Core.Models;
+using LiteDB;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+public sealed class LiteDbContextTests
+{
+    [Fact]
+    public void MigratesLegacyEnrollmentAndSubjectTextWithoutLosingData()
+    {
+        using var temp = new TempDatabase();
+        var groupId = Guid.NewGuid();
+        var studentId = Guid.NewGuid();
+        var membershipId = Guid.NewGuid();
+        var examId = Guid.NewGuid();
+        var gradeId = Guid.NewGuid();
+        var entryId = Guid.NewGuid();
+        var addedOn = new DateOnly(2026, 6, 23);
+
+        using (var legacy = new LiteDatabase(temp.Path))
+        {
+            legacy.GetCollection<Subject>("subjects").Insert(new Subject { Name = "mathematik" });
+            legacy.GetCollection<BsonDocument>("groups").Insert(new BsonDocument
+            {
+                ["_id"] = groupId,
+                [nameof(LearningGroup.Name)] = "Mathe G",
+                ["Subject"] = "Mathematik",
+                [nameof(LearningGroup.SchoolYear)] = "2025/26",
+            });
+            legacy.GetCollection<BsonDocument>("enrollments").Insert(new BsonDocument
+            {
+                ["_id"] = membershipId,
+                [nameof(GroupMembership.StudentId)] = studentId,
+                [nameof(GroupMembership.GroupId)] = groupId,
+                ["SchoolYear"] = "2025/26",
+                ["EnrolledAt"] = BsonMapper.Global.Serialize(addedOn),
+            });
+            legacy.GetCollection<BsonDocument>("exams").Insert(new BsonDocument
+            {
+                ["_id"] = examId,
+                [nameof(Exam.GroupId)] = groupId,
+                ["Subject"] = "Mathematik",
+            });
+            legacy.GetCollection<BsonDocument>("grades").Insert(new BsonDocument
+            {
+                ["_id"] = gradeId,
+                [nameof(Grade.StudentId)] = studentId,
+                [nameof(Grade.GroupId)] = groupId,
+                ["SchoolYear"] = "2025/26",
+            });
+            legacy.GetCollection<BsonDocument>("participation").Insert(new BsonDocument
+            {
+                ["_id"] = entryId,
+                [nameof(ParticipationEntry.SessionId)] = Guid.NewGuid(),
+                [nameof(ParticipationEntry.StudentId)] = studentId,
+                ["GroupId"] = groupId,
+                ["Date"] = BsonMapper.Global.Serialize(addedOn),
+            });
+        }
+
+        using (var context = new LiteDbContext(temp.Path))
+        {
+            var group = context.Groups.FindById(groupId);
+            var membership = context.Memberships.FindById(membershipId);
+            Assert.NotNull(group);
+            Assert.NotNull(group.SubjectId);
+            Assert.Equal("mathematik", context.Subjects.FindById(group.SubjectId.Value)?.Name);
+            Assert.Equal(1, context.Subjects.Count());
+            Assert.NotNull(membership);
+            Assert.Equal(addedOn, membership.AddedOn);
+        }
+
+        using var migrated = new LiteDatabase(temp.Path);
+        Assert.DoesNotContain("enrollments", migrated.GetCollectionNames());
+        Assert.Contains("group_memberships", migrated.GetCollectionNames());
+        var rawGroup = migrated.GetCollection<BsonDocument>("groups").FindById(groupId);
+        var rawMembership = migrated.GetCollection<BsonDocument>("group_memberships").FindById(membershipId);
+        var rawExam = migrated.GetCollection<BsonDocument>("exams").FindById(examId);
+        var rawGrade = migrated.GetCollection<BsonDocument>("grades").FindById(gradeId);
+        var rawEntry = migrated.GetCollection<BsonDocument>("participation").FindById(entryId);
+        Assert.False(rawGroup.ContainsKey("Subject"));
+        Assert.False(rawMembership.ContainsKey("SchoolYear"));
+        Assert.False(rawMembership.ContainsKey("EnrolledAt"));
+        Assert.True(rawMembership.ContainsKey(nameof(GroupMembership.AddedOn)));
+        Assert.False(rawExam.ContainsKey("Subject"));
+        Assert.False(rawGrade.ContainsKey("SchoolYear"));
+        Assert.False(rawEntry.ContainsKey("GroupId"));
+        Assert.False(rawEntry.ContainsKey("Date"));
+    }
+
+    [Fact]
+    public void RejectsASecondMembershipForTheSameStudentAndGroup()
+    {
+        using var temp = new TempDatabase();
+        using var context = new LiteDbContext(temp.Path);
+        var studentId = Guid.NewGuid();
+        var groupId = Guid.NewGuid();
+        context.Memberships.Insert(new GroupMembership { StudentId = studentId, GroupId = groupId });
+
+        Assert.Throws<LiteException>(() => context.Memberships.Insert(
+            new GroupMembership { StudentId = studentId, GroupId = groupId }));
+    }
+
+    [Fact]
+    public void RejectsASecondExamResultForTheSameStudentAndExam()
+    {
+        using var temp = new TempDatabase();
+        using var context = new LiteDbContext(temp.Path);
+        var studentId = Guid.NewGuid();
+        var examId = Guid.NewGuid();
+        context.ExamResults.Insert(new ExamResult { StudentId = studentId, ExamId = examId });
+
+        Assert.Throws<LiteException>(() => context.ExamResults.Insert(
+            new ExamResult { StudentId = studentId, ExamId = examId }));
+    }
+
+    private sealed class TempDatabase : IDisposable
+    {
+        private readonly string _directory = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(), $"lehrerapp-tests-{Guid.NewGuid():N}");
+        public string Path { get; }
+
+        public TempDatabase()
+        {
+            Directory.CreateDirectory(_directory);
+            Path = System.IO.Path.Combine(_directory, "test.db");
+        }
+
+        public void Dispose()
+        {
+            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
+        }
+    }
+}

LehrerApp.Data/LiteDbContext.cs

@@ -22,7 +22,7 @@ public class LiteDbContext : IDisposable
 
     public ILiteCollection<Student>       Students      => _db.GetCollection<Student>("students");
     public ILiteCollection<LearningGroup> Groups        => _db.GetCollection<LearningGroup>("groups");
-    public ILiteCollection<Enrollment>    Enrollments   => _db.GetCollection<Enrollment>("enrollments");
+    public ILiteCollection<GroupMembership> Memberships => _db.GetCollection<GroupMembership>("group_memberships");
     public ILiteCollection<Exam>          Exams         => _db.GetCollection<Exam>("exams");
     public ILiteCollection<ExamResult>    ExamResults   => _db.GetCollection<ExamResult>("exam_results");
     public ILiteCollection<Grade>         Grades        => _db.GetCollection<Grade>("grades");
@@ -42,7 +42,11 @@ public class LiteDbContext : IDisposable
 
     private void MigrateExistingData()
     {
+        MigrateMemberships();
+        RemoveRedundantLegacyFields();
+
         var groups = _db.GetCollection<BsonDocument>("groups");
+        var subjects = _db.GetCollection<Subject>("subjects");
         var missingArchiveState = groups.FindAll()
             .Where(g => !g.ContainsKey(nameof(LearningGroup.IsActive)))
             .ToList();
@@ -51,6 +55,83 @@ public class LiteDbContext : IDisposable
             group[nameof(LearningGroup.IsActive)] = true;
             groups.Update(group);
         }
+
+        // Subject war anfangs nur als Text in der Gruppe gespeichert. Alte
+        // Werte werden einmalig an die Fachstammdaten angebunden und danach
+        // entfernt, damit es nur noch eine führende Fachbezeichnung gibt.
+        foreach (var group in groups.FindAll().ToList())
+        {
+            var changed = false;
+            var legacyName = group.TryGetValue("Subject", out var value) && value.IsString
+                ? value.AsString.Trim()
+                : "";
+            var hasSubjectId = group.TryGetValue(nameof(LearningGroup.SubjectId), out var id)
+                               && id.IsGuid && subjects.FindById(id.AsGuid) is not null;
+
+            if (!hasSubjectId && legacyName.Length > 0)
+            {
+                var subject = subjects.FindAll().FirstOrDefault(s =>
+                    string.Equals(s.Name.Trim(), legacyName, StringComparison.OrdinalIgnoreCase));
+                if (subject is null)
+                {
+                    subject = new Subject { Name = legacyName };
+                    subjects.Insert(subject);
+                }
+                group[nameof(LearningGroup.SubjectId)] = subject.Id;
+                changed = true;
+            }
+            else if (!hasSubjectId && group.ContainsKey(nameof(LearningGroup.SubjectId)))
+            {
+                group[nameof(LearningGroup.SubjectId)] = BsonValue.Null;
+                changed = true;
+            }
+
+            changed |= group.Remove("Subject");
+            if (changed) groups.Update(group);
+        }
+    }
+
+    private void MigrateMemberships()
+    {
+        const string oldName = "enrollments";
+        const string newName = "group_memberships";
+        var names = _db.GetCollectionNames().ToHashSet(StringComparer.OrdinalIgnoreCase);
+        if (names.Contains(oldName) && !names.Contains(newName))
+            _db.RenameCollection(oldName, newName);
+
+        var memberships = _db.GetCollection<BsonDocument>(newName);
+        foreach (var membership in memberships.FindAll().ToList())
+        {
+            var changed = false;
+            if (!membership.ContainsKey(nameof(GroupMembership.AddedOn)) &&
+                membership.TryGetValue("EnrolledAt", out var enrolledAt))
+            {
+                membership[nameof(GroupMembership.AddedOn)] = enrolledAt;
+                changed = true;
+            }
+            changed |= membership.Remove("EnrolledAt");
+            changed |= membership.Remove("SchoolYear");
+            if (changed) memberships.Update(membership);
+        }
+    }
+
+    private void RemoveRedundantLegacyFields()
+    {
+        RemoveFields("exams", "Subject");
+        RemoveFields("units", "Subject", "SchoolYear");
+        RemoveFields("grades", "SchoolYear");
+        RemoveFields("participation", "GroupId", "Date");
+    }
+
+    private void RemoveFields(string collectionName, params string[] fieldNames)
+    {
+        var collection = _db.GetCollection<BsonDocument>(collectionName);
+        foreach (var document in collection.FindAll().ToList())
+        {
+            var changed = false;
+            foreach (var fieldName in fieldNames) changed |= document.Remove(fieldName);
+            if (changed) collection.Update(document);
+        }
     }
 
     private void EnsureIndexes()
@@ -59,13 +140,16 @@ public class LiteDbContext : IDisposable
         Students.EnsureIndex(x => x.IsActive);
         Groups.EnsureIndex(x => x.SchoolYear);
         Groups.EnsureIndex(x => x.IsActive);
-        Enrollments.EnsureIndex(x => x.StudentId);
-        Enrollments.EnsureIndex(x => x.GroupId);
-        Enrollments.EnsureIndex(x => x.SchoolYear);
+        Memberships.EnsureIndex(x => x.StudentId);
+        Memberships.EnsureIndex(x => x.GroupId);
+        Memberships.EnsureIndex("ux_student_group",
+            BsonExpression.Create("STRING($.StudentId) + ':' + STRING($.GroupId)"), unique: true);
         Exams.EnsureIndex(x => x.GroupId);
         Exams.EnsureIndex(x => x.Status);
         ExamResults.EnsureIndex(x => x.ExamId);
         ExamResults.EnsureIndex(x => x.StudentId);
+        ExamResults.EnsureIndex("ux_exam_student",
+            BsonExpression.Create("STRING($.ExamId) + ':' + STRING($.StudentId)"), unique: true);
         Grades.EnsureIndex(x => x.StudentId);
         Grades.EnsureIndex(x => x.GroupId);
         GradingKeyTemplates.EnsureIndex(x => x.GradingSystem);
@@ -80,8 +164,11 @@ public class LiteDbContext : IDisposable
         ParticipationSessions.EnsureIndex(x => x.Date);
         ParticipationEntries.EnsureIndex(x => x.SessionId);
         ParticipationEntries.EnsureIndex(x => x.StudentId);
+        ParticipationEntries.EnsureIndex("ux_session_student",
+            BsonExpression.Create("STRING($.SessionId) + ':' + STRING($.StudentId)"), unique: true);
         ParticipationAspects.EnsureIndex(x => x.GroupId);
         Subjects.EnsureIndex(x => x.Name);
+        Subjects.EnsureIndex("ux_subject_name", BsonExpression.Create("LOWER(TRIM($.Name))"), unique: true);
         CompetencyDomains.EnsureIndex(x => x.SubjectId);
         CompetencyDomains.EnsureIndex(x => x.GradeLevel);
     }

LehrerApp.Data/Repositories/AllRepositories.cs

@@ -9,10 +9,10 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
     public List<Student> GetAll(bool includeInactive = false) =>
         (includeInactive ? db.Students.FindAll() : db.Students.Find(s => s.IsActive))
         .OrderBy(s => s.LastName).ToList();
-    public List<Student> GetByGroup(Guid groupId, string schoolYear)
+    public List<Student> GetByGroup(Guid groupId)
     {
-        var ids = db.Enrollments
-            .Find(e => e.GroupId == groupId && e.SchoolYear == schoolYear)
+        var ids = db.Memberships
+            .Find(e => e.GroupId == groupId)
             .Select(e => e.StudentId).ToHashSet();
         return db.Students.Find(s => ids.Contains(s.Id)).OrderBy(s => s.LastName).ToList();
     }
@@ -31,11 +31,17 @@ public class GroupRepository(LiteDbContext db) : IGroupRepository
             ? db.Groups.Find(g => g.SchoolYear == schoolYear)
             : db.Groups.Find(g => g.SchoolYear == schoolYear && g.IsActive))
         .OrderBy(g => g.Name).ToList();
-    public void Save(LearningGroup g) { g.UpdatedAt = DateTime.UtcNow; db.Groups.Upsert(g); }
+    public void Save(LearningGroup g)
+    {
+        if (g.SubjectId is Guid subjectId && db.Subjects.FindById(subjectId) is null)
+            throw new InvalidOperationException("Das gewählte Fach existiert nicht mehr.");
+        g.UpdatedAt = DateTime.UtcNow;
+        db.Groups.Upsert(g);
+    }
     public void Delete(Guid id)
     {
-        foreach (var enrollment in db.Enrollments.Find(e => e.GroupId == id).ToList())
-            db.Enrollments.Delete(enrollment.Id);
+        foreach (var membership in db.Memberships.Find(e => e.GroupId == id).ToList())
+            db.Memberships.Delete(membership.Id);
 
         foreach (var exam in db.Exams.Find(e => e.GroupId == id).ToList())
         {
@@ -71,16 +77,22 @@ public class GroupRepository(LiteDbContext db) : IGroupRepository
     }
 }
 
-public class EnrollmentRepository(LiteDbContext db) : IEnrollmentRepository
+public class GroupMembershipRepository(LiteDbContext db) : IGroupMembershipRepository
 {
-    public List<Enrollment> GetByStudent(Guid id) =>
-        db.Enrollments.Find(e => e.StudentId == id).ToList();
-    public List<Enrollment> GetByGroup(Guid id) =>
-        db.Enrollments.Find(e => e.GroupId == id).ToList();
-    public List<Enrollment> GetByGroupAndYear(Guid groupId, string sy) =>
-        db.Enrollments.Find(e => e.GroupId == groupId && e.SchoolYear == sy).ToList();
-    public void Save(Enrollment e) => db.Enrollments.Upsert(e);
-    public void Delete(Guid id) => db.Enrollments.Delete(id);
+    public List<GroupMembership> GetByStudent(Guid id) =>
+        db.Memberships.Find(e => e.StudentId == id).ToList();
+    public List<GroupMembership> GetByGroup(Guid id) =>
+        db.Memberships.Find(e => e.GroupId == id).ToList();
+    public GroupMembership? GetByStudentAndGroup(Guid studentId, Guid groupId) =>
+        db.Memberships.FindOne(e => e.StudentId == studentId && e.GroupId == groupId);
+    public void Save(GroupMembership membership)
+    {
+        var existing = GetByStudentAndGroup(membership.StudentId, membership.GroupId);
+        if (existing is not null && existing.Id != membership.Id)
+            throw new InvalidOperationException("Der Schüler ist dieser Lerngruppe bereits zugeordnet.");
+        db.Memberships.Upsert(membership);
+    }
+    public void Delete(Guid id) => db.Memberships.Delete(id);
 }
 
 public class ExamRepository(LiteDbContext db) : IExamRepository
@@ -240,10 +252,25 @@ public class SubjectRepository(LiteDbContext db) : ISubjectRepository
 {
     public List<Subject> GetAll() => db.Subjects.FindAll().OrderBy(s => s.Name).ToList();
     public Subject? GetById(Guid id) => db.Subjects.FindById(id);
-    public Subject? GetByName(string name) =>
-        db.Subjects.FindOne(s => s.Name == name);
-    public void Save(Subject s) { s.UpdatedAt = DateTime.UtcNow; db.Subjects.Upsert(s); }
-    public void Delete(Guid id) => db.Subjects.Delete(id);
+    public Subject? GetByName(string name) => db.Subjects.FindAll().FirstOrDefault(s =>
+        string.Equals(s.Name.Trim(), name.Trim(), StringComparison.OrdinalIgnoreCase));
+    public void Save(Subject s)
+    {
+        s.Name = s.Name.Trim();
+        s.ShortName = s.ShortName.Trim();
+        if (s.Name.Length == 0) throw new ArgumentException("Der Fachname darf nicht leer sein.");
+        var duplicate = GetByName(s.Name);
+        if (duplicate is not null && duplicate.Id != s.Id)
+            throw new InvalidOperationException("Ein Fach mit diesem Namen existiert bereits.");
+        s.UpdatedAt = DateTime.UtcNow;
+        db.Subjects.Upsert(s);
+    }
+    public void Delete(Guid id)
+    {
+        if (db.Groups.Exists(g => g.SubjectId == id) || db.CompetencyDomains.Exists(d => d.SubjectId == id))
+            throw new InvalidOperationException("Das Fach wird noch von einer Lerngruppe oder einem Kompetenzkatalog verwendet.");
+        db.Subjects.Delete(id);
+    }
 }
 
 public class CompetencyDomainRepository(LiteDbContext db) : ICompetencyDomainRepository

LehrerApp.Desktop/AppBootstrapper.cs

@@ -44,7 +44,7 @@ public static class AppBootstrapper
         // ── Repositories ──────────────────────────────────────────────────────
         services.AddSingleton<IStudentRepository,       StudentRepository>();
         services.AddSingleton<IGroupRepository,         GroupRepository>();
-        services.AddSingleton<IEnrollmentRepository,    EnrollmentRepository>();
+        services.AddSingleton<IGroupMembershipRepository, GroupMembershipRepository>();
         services.AddSingleton<IExamRepository,          ExamRepository>();
         services.AddSingleton<IExamResultRepository,    ExamResultRepository>();
         services.AddSingleton<IGradeRepository,         GradeRepository>();

LehrerApp.Desktop/ViewModels/DashboardViewModel.cs

@@ -13,6 +13,7 @@ public partial class DashboardViewModel : ObservableObject
     private static readonly CultureInfo De = new("de-DE");
 
     private readonly IGroupRepository _groups;
+    private readonly ISubjectRepository _subjects;
     private readonly ILessonRepository _lessons;
     private readonly IExamRepository _exams;
     private readonly IWorkTaskRepository _tasks;
@@ -34,10 +35,10 @@ public partial class DashboardViewModel : ObservableObject
     // Navigation-Callback – wird von App.axaml.cs verdrahtet
     public Action<Guid>? OnNavigateToGroup { get; set; }
 
-    public DashboardViewModel(IGroupRepository groups, ILessonRepository lessons,
+    public DashboardViewModel(IGroupRepository groups, ISubjectRepository subjects, ILessonRepository lessons,
         IExamRepository exams, IWorkTaskRepository tasks, SchoolYearService sy)
     {
-        _groups = groups; _lessons = lessons; _exams = exams; _tasks = tasks; _sy = sy;
+        _groups = groups; _subjects = subjects; _lessons = lessons; _exams = exams; _tasks = tasks; _sy = sy;
         Load();
     }
 
@@ -70,7 +71,12 @@ public partial class DashboardViewModel : ObservableObject
 
         CurrentGroups.Clear();
         foreach (var g in groups.Values.OrderBy(g => g.Name))
-            CurrentGroups.Add(new() { GroupId = g.Id, Name = g.Name, Subject = g.Subject ?? "" });
+            CurrentGroups.Add(new()
+            {
+                GroupId = g.Id,
+                Name = g.Name,
+                Subject = g.SubjectId is Guid subjectId ? _subjects.GetById(subjectId)?.Name ?? "" : "",
+            });
 
         CalendarMonth = FirstOfMonth(now);
         LoadCalendar();

LehrerApp.Desktop/ViewModels/Groups/ExamGradingViewModels.cs

@@ -24,24 +24,24 @@ public partial class ExamGradingDialogViewModel : ObservableObject
     public ObservableCollection<ExamResultRow> Rows { get; } = [];
 
     public ExamGradingDialogViewModel(IExamResultRepository results, IStudentRepository students,
-        IEnrollmentRepository enrollments, GradingService grading, Exam exam, Guid groupId, string schoolYear)
+        IGroupMembershipRepository memberships, GradingService grading, Exam exam, Guid groupId)
     {
         _results = results; _grading = grading; _exam = exam;
         Tasks = exam.Tasks.OrderBy(t => t.Nr).ToList();
         _examMaxPoints = Tasks.Sum(t => t.MaxPoints);
 
-        var enrolled = students.GetByGroup(groupId, schoolYear);
-        var enrollmentList = enrollments.GetByGroupAndYear(groupId, schoolYear);
+        var enrolled = students.GetByGroup(groupId);
+        var membershipList = memberships.GetByGroup(groupId);
         var existing = results.GetByExam(exam.Id).ToDictionary(r => r.StudentId);
 
         foreach (var s in enrolled.OrderBy(s => s.LastName).ThenBy(s => s.FirstName))
         {
-            var enrollment = enrollmentList.FirstOrDefault(e => e.StudentId == s.Id);
-            if (enrollment is not null && !IsEnrolledAtDate(enrollment, exam.Date)) continue;
+            var membership = membershipList.FirstOrDefault(e => e.StudentId == s.Id);
+            if (membership is not null && !IsMemberAtDate(membership, exam.Date)) continue;
 
             // Niveau-Klausur: nur Schüler mit passendem Niveau zeigen. Klausuren ohne
             // Niveau-Zuordnung gelten weiterhin für die ganze Gruppe.
-            if (exam.Niveau.HasValue && enrollment?.Niveau != exam.Niveau) continue;
+            if (exam.Niveau.HasValue && membership?.Niveau != exam.Niveau) continue;
 
             existing.TryGetValue(s.Id, out var result);
             var row = new ExamResultRow(s.Id, s.FullName, Tasks, result, _exam.GradingKey, _examMaxPoints, _grading);
@@ -52,12 +52,12 @@ public partial class ExamGradingDialogViewModel : ObservableObject
 
     private void SaveRow(ExamResultRow row) => _results.Save(row.ToModel(_exam.Id));
 
-    private static bool IsEnrolledAtDate(Enrollment e, DateOnly date) => e.Period switch
+    private static bool IsMemberAtDate(GroupMembership membership, DateOnly date) => membership.Period switch
     {
-        EnrollmentPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
-        EnrollmentPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
-        EnrollmentPeriod.Custom => (e.JoinedAt is null || date >= e.JoinedAt.Value)
-                                 && (e.LeftAt  is null || date <= e.LeftAt.Value),
+        MembershipPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
+        MembershipPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
+        MembershipPeriod.Custom => (membership.JoinedAt is null || date >= membership.JoinedAt.Value)
+                                && (membership.LeftAt  is null || date <= membership.LeftAt.Value),
         _                       => true,
     };
 }

LehrerApp.Desktop/ViewModels/Groups/ExamViewModels.cs

@@ -289,7 +289,6 @@ public partial class ExamDialogViewModel : ObservableObject
         Result = _editingExam ?? new Exam { GroupId = _groupId };
         Result.Title      = Title.Trim();
         Result.Date       = date;
-        Result.Subject    = _subjectName.Trim();
         Result.ExamNumber = ExamNumber;
         Result.Notes      = string.IsNullOrWhiteSpace(Notes) ? null : Notes.Trim();
         Result.ReturnedAt = returnedAt;

LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs

@@ -12,7 +12,7 @@ namespace LehrerApp.Desktop.ViewModels.Groups;
 public partial class GroupListViewModel : ObservableObject
 {
     private readonly IGroupRepository _groups;
-    private readonly SchoolYearService _sy;
+    private readonly ISubjectRepository _subjects;
 
     public Action<Guid, int>? OnNavigateToDetail { get; set; }
     public Func<Task>? OnAddGroup { get; set; }
@@ -37,9 +37,9 @@ public partial class GroupListViewModel : ObservableObject
     public ObservableCollection<string> SchoolYears { get; } = [];
     public ObservableCollection<GroupListItem> Groups { get; } = [];
 
-    public GroupListViewModel(IGroupRepository groups, SchoolYearService sy)
+    public GroupListViewModel(IGroupRepository groups, ISubjectRepository subjects, SchoolYearService sy)
     {
-        _groups = groups; _sy = sy;
+        _groups = groups; _subjects = subjects;
         foreach (var y in sy.RecentSchoolYears()) SchoolYears.Add(y);
         SelectedSchoolYear = sy.CurrentSchoolYear();
     }
@@ -63,11 +63,13 @@ public partial class GroupListViewModel : ObservableObject
         Groups.Clear();
         var all = _groups.GetBySchoolYear(SelectedSchoolYear, includeInactive: ShowArchived)
             .Where(g => g.IsActive != ShowArchived);
+        var subjectNames = _subjects.GetAll().ToDictionary(s => s.Id, s => s.Name);
         var filtered = string.IsNullOrWhiteSpace(SearchText) ? all
             : all.Where(g => g.Name.Contains(SearchText, StringComparison.OrdinalIgnoreCase)
-                          || (g.Subject?.Contains(SearchText, StringComparison.OrdinalIgnoreCase) ?? false));
+                          || (g.SubjectId is Guid id && subjectNames.GetValueOrDefault(id, "")
+                              .Contains(SearchText, StringComparison.OrdinalIgnoreCase)));
         foreach (var g in filtered.OrderBy(g => g.Name))
-            Groups.Add(new GroupListItem(g));
+            Groups.Add(new GroupListItem(g, g.SubjectId is Guid id ? subjectNames.GetValueOrDefault(id, "") : ""));
         SelectedGroup = Groups.FirstOrDefault(g => g.Id == selectedId);
         OnPropertyChanged(nameof(ListSummary));
         OnPropertyChanged(nameof(HasNoGroups));
@@ -136,15 +138,15 @@ public class GroupListItem
     public bool IsActive { get; }
     public string ArchiveActionLabel => IsActive ? "Archivieren" : "Wieder aktivieren";
 
-    public GroupListItem(LearningGroup g)
+    public GroupListItem(LearningGroup g, string subjectName)
     {
         Id           = g.Id;
         IsActive     = g.IsActive;
         Name         = g.Name;
-        Subject      = g.Subject ?? "";
+        Subject      = subjectName;
         TypeLabel    = g.Type == GroupType.Class ? "Klasse" : "Kurs";
         GradingLabel = g.GradingSystem == GradingSystem.Grades1To6 ? "1–6" : "0–15";
-        DisplayName  = string.IsNullOrEmpty(g.Subject) ? g.Name : $"{g.Name} · {g.Subject}";
+        DisplayName  = string.IsNullOrEmpty(subjectName) ? g.Name : $"{g.Name} · {subjectName}";
         Subtitle     = $"{TypeLabel} · Stufe {g.GradeLevel} · Noten {GradingLabel} · {g.SchoolYear}";
     }
 }
@@ -155,7 +157,8 @@ public partial class GroupDetailViewModel : ObservableObject
 {
     private readonly IGroupRepository _groups;
     private readonly IStudentRepository _students;
-    private readonly IEnrollmentRepository _enrollments;
+    private readonly IGroupMembershipRepository _memberships;
+    private readonly ISubjectRepository _subjects;
     private readonly IExamRepository _exams;
     private readonly IGradeRepository _grades;
 
@@ -171,6 +174,7 @@ public partial class GroupDetailViewModel : ObservableObject
     // bevor LoadGroup() läuft (siehe MainWindowViewModel.NavigateToGroupDetail), Group ist dann
     // kurzzeitig null — ein verschachtelter Pfad würde dafür jedes Mal einen Binding-Fehler loggen.
     public bool IsDifferentiated => Group?.IsDifferentiated ?? false;
+    public string SubjectName { get; private set; } = "";
 
     partial void OnGroupChanged(LearningGroup? value) => OnPropertyChanged(nameof(IsDifferentiated));
 
@@ -187,10 +191,11 @@ public partial class GroupDetailViewModel : ObservableObject
     public Func<Exam, Task>? OnEvaluateExam { get; set; }
 
     public GroupDetailViewModel(IGroupRepository groups, IStudentRepository students,
-        IEnrollmentRepository enrollments, IExamRepository exams, IGradeRepository grades,
+        IGroupMembershipRepository memberships, ISubjectRepository subjects,
+        IExamRepository exams, IGradeRepository grades,
         ParticipationTabViewModel participationTab)
     {
-        _groups = groups; _students = students; _enrollments = enrollments;
+        _groups = groups; _students = students; _memberships = memberships; _subjects = subjects;
         _exams = exams; _grades = grades;
         ParticipationTab = participationTab;
     }
@@ -199,6 +204,9 @@ public partial class GroupDetailViewModel : ObservableObject
     {
         Group = _groups.GetById(id);
         if (Group is null) return;
+        SubjectName = Group.SubjectId is Guid subjectId
+            ? _subjects.GetById(subjectId)?.Name ?? ""
+            : "";
         GroupTitle    = Group.Name;
         GroupSubtitle = $"{Group.SchoolYear} · " +
                         $"{(Group.Type == GroupType.Class ? "Klasse" : "Kurs")} · " +
@@ -221,14 +229,13 @@ public partial class GroupDetailViewModel : ObservableObject
     {
         if (Group is null) return;
         Students.Clear();
-        var enrolled    = _students.GetByGroup(Group.Id, Group.SchoolYear);
-        var enrollments = _enrollments.GetByGroupAndYear(Group.Id, Group.SchoolYear)
-                          .ToDictionary(e => e.StudentId);
+        var enrolled = _students.GetByGroup(Group.Id);
+        var memberships = _memberships.GetByGroup(Group.Id).ToDictionary(e => e.StudentId);
         StudentCount = enrolled.Count;
         foreach (var s in enrolled)
         {
-            enrollments.TryGetValue(s.Id, out var enr);
-            var summary = new StudentSummary(s, enr) { OnChanged = SaveStudentNiveau };
+            memberships.TryGetValue(s.Id, out var membership);
+            var summary = new StudentSummary(s, membership) { OnChanged = SaveStudentNiveau };
             Students.Add(summary);
         }
     }
@@ -236,11 +243,10 @@ public partial class GroupDetailViewModel : ObservableObject
     private void SaveStudentNiveau(StudentSummary summary)
     {
         if (Group is null) return;
-        var enrollment = _enrollments.GetByGroupAndYear(Group.Id, Group.SchoolYear)
-            .FirstOrDefault(e => e.StudentId == summary.Id);
-        if (enrollment is null) return;
-        enrollment.Niveau = summary.Niveau;
-        _enrollments.Save(enrollment);
+        var membership = _memberships.GetByStudentAndGroup(summary.Id, Group.Id);
+        if (membership is null) return;
+        membership.Niveau = summary.Niveau;
+        _memberships.Save(membership);
     }
 
     [RelayCommand]
@@ -259,10 +265,9 @@ public partial class GroupDetailViewModel : ObservableObject
     private void RemoveStudent()
     {
         if (Group is null || SelectedStudent is null) return;
-        var enrollment = _enrollments.GetByGroupAndYear(Group.Id, Group.SchoolYear)
-            .FirstOrDefault(e => e.StudentId == SelectedStudent.Id);
-        if (enrollment is null) return;
-        _enrollments.Delete(enrollment.Id);
+        var membership = _memberships.GetByStudentAndGroup(SelectedStudent.Id, Group.Id);
+        if (membership is null) return;
+        _memberships.Delete(membership.Id);
         LoadStudents();
         SelectedStudent = null;
         ParticipationTab.RefreshCurrentGrid();
@@ -422,18 +427,18 @@ public partial class StudentSummary : ObservableObject
 
     public Action<StudentSummary>? OnChanged { get; set; }
 
-    public StudentSummary(Core.Models.Student s, Enrollment? e)
+    public StudentSummary(Core.Models.Student s, GroupMembership? membership)
     {
         Id     = s.Id;
         FullName = s.FullName;
-        PeriodLabel = e?.Period switch
+        PeriodLabel = membership?.Period switch
         {
-            EnrollmentPeriod.H1Only => "H1",
-            EnrollmentPeriod.H2Only => "H2",
-            EnrollmentPeriod.Custom => BuildCustomLabel(e),
+            MembershipPeriod.H1Only => "H1",
+            MembershipPeriod.H2Only => "H2",
+            MembershipPeriod.Custom => BuildCustomLabel(membership),
             _                       => "",
         };
-        _niveau = e?.Niveau;
+        _niveau = membership?.Niveau;
     }
 
     partial void OnNiveauChanged(Niveau? value)
@@ -442,11 +447,11 @@ public partial class StudentSummary : ObservableObject
         OnChanged?.Invoke(this);
     }
 
-    private static string BuildCustomLabel(Enrollment e)
+    private static string BuildCustomLabel(GroupMembership membership)
     {
-        if (e.JoinedAt.HasValue && e.LeftAt.HasValue) return $"{e.JoinedAt:dd.MM.}–{e.LeftAt:dd.MM.}";
-        if (e.JoinedAt.HasValue) return $"ab {e.JoinedAt:dd.MM.}";
-        if (e.LeftAt.HasValue)   return $"bis {e.LeftAt:dd.MM.}";
+        if (membership.JoinedAt.HasValue && membership.LeftAt.HasValue) return $"{membership.JoinedAt:dd.MM.}–{membership.LeftAt:dd.MM.}";
+        if (membership.JoinedAt.HasValue) return $"ab {membership.JoinedAt:dd.MM.}";
+        if (membership.LeftAt.HasValue)   return $"bis {membership.LeftAt:dd.MM.}";
         return "Datum";
     }
 }
@@ -491,35 +496,34 @@ public class ExamSummary
 public partial class AddStudentToGroupDialogViewModel : ObservableObject
 {
     private readonly IStudentRepository    _students;
-    private readonly IEnrollmentRepository _enrollments;
+    private readonly IGroupMembershipRepository _memberships;
     private readonly Guid   _groupId;
-    private readonly string _schoolYear;
 
     [ObservableProperty] private string            _searchText        = "";
     [ObservableProperty] private StudentPickerItem? _selectedStudent;
     [ObservableProperty] private string            _validationMessage  = "";
-    [ObservableProperty] private EnrollmentPeriod  _period             = EnrollmentPeriod.FullYear;
+    [ObservableProperty] private MembershipPeriod  _period             = MembershipPeriod.FullYear;
     [ObservableProperty] private string            _joinedAtText       = "";
     [ObservableProperty] private string            _leftAtText         = "";
 
-    public bool IsFullYear { get => Period == EnrollmentPeriod.FullYear; set { if (value) Period = EnrollmentPeriod.FullYear; } }
-    public bool IsH1Only   { get => Period == EnrollmentPeriod.H1Only;   set { if (value) Period = EnrollmentPeriod.H1Only; } }
-    public bool IsH2Only   { get => Period == EnrollmentPeriod.H2Only;   set { if (value) Period = EnrollmentPeriod.H2Only; } }
-    public bool IsCustom   { get => Period == EnrollmentPeriod.Custom;   set { if (value) Period = EnrollmentPeriod.Custom; } }
-    public bool IsCustomPeriod => Period == EnrollmentPeriod.Custom;
+    public bool IsFullYear { get => Period == MembershipPeriod.FullYear; set { if (value) Period = MembershipPeriod.FullYear; } }
+    public bool IsH1Only   { get => Period == MembershipPeriod.H1Only;   set { if (value) Period = MembershipPeriod.H1Only; } }
+    public bool IsH2Only   { get => Period == MembershipPeriod.H2Only;   set { if (value) Period = MembershipPeriod.H2Only; } }
+    public bool IsCustom   { get => Period == MembershipPeriod.Custom;   set { if (value) Period = MembershipPeriod.Custom; } }
+    public bool IsCustomPeriod => Period == MembershipPeriod.Custom;
 
     public ObservableCollection<StudentPickerItem> AvailableStudents { get; } = [];
-    public Enrollment? Result { get; private set; }
+    public GroupMembership? Result { get; private set; }
 
     public AddStudentToGroupDialogViewModel(IStudentRepository students,
-        IEnrollmentRepository enrollments, Guid groupId, string schoolYear)
+        IGroupMembershipRepository memberships, Guid groupId)
     {
-        _students = students; _enrollments = enrollments;
-        _groupId = groupId; _schoolYear = schoolYear;
+        _students = students; _memberships = memberships;
+        _groupId = groupId;
         LoadAvailableStudents();
     }
 
-    partial void OnPeriodChanged(EnrollmentPeriod value)
+    partial void OnPeriodChanged(MembershipPeriod value)
     {
         OnPropertyChanged(nameof(IsFullYear));
         OnPropertyChanged(nameof(IsH1Only));
@@ -532,11 +536,11 @@ public partial class AddStudentToGroupDialogViewModel : ObservableObject
 
     private void LoadAvailableStudents()
     {
-        var alreadyEnrolled = _enrollments.GetByGroupAndYear(_groupId, _schoolYear)
+        var alreadyAssigned = _memberships.GetByGroup(_groupId)
             .Select(e => e.StudentId).ToHashSet();
         var all = _students.GetAll();
         var available = all
-            .Where(s => !alreadyEnrolled.Contains(s.Id))
+            .Where(s => !alreadyAssigned.Contains(s.Id))
             .Where(s => string.IsNullOrWhiteSpace(SearchText) ||
                         s.FullName.Contains(SearchText, StringComparison.OrdinalIgnoreCase));
         AvailableStudents.Clear();
@@ -549,7 +553,7 @@ public partial class AddStudentToGroupDialogViewModel : ObservableObject
         if (SelectedStudent is null) { ValidationMessage = "Bitte einen Schüler auswählen."; return; }
 
         DateOnly? joinedAt = null, leftAt = null;
-        if (Period == EnrollmentPeriod.Custom)
+        if (Period == MembershipPeriod.Custom)
         {
             if (!string.IsNullOrWhiteSpace(JoinedAtText) &&
                 DateOnly.TryParseExact(JoinedAtText, "dd.MM.yyyy",
@@ -561,16 +565,15 @@ public partial class AddStudentToGroupDialogViewModel : ObservableObject
                 leftAt = l;
         }
 
-        Result = new Enrollment
+        Result = new GroupMembership
         {
             StudentId  = SelectedStudent.Id,
             GroupId    = _groupId,
-            SchoolYear = _schoolYear,
             Period     = Period,
             JoinedAt   = joinedAt,
             LeftAt     = leftAt,
         };
-        _enrollments.Save(Result);
+        _memberships.Save(Result);
     }
 }
 
@@ -587,11 +590,8 @@ public partial class AddGroupDialogViewModel : ObservableObject
 {
     private readonly IGroupRepository   _groups;
     private readonly ISubjectRepository _subjects;
-    private readonly SchoolYearService  _sy;
-    private readonly IEnrollmentRepository _enrollments;
     private List<Subject> _allSubjects = [];
     private LearningGroup? _editingGroup;
-    private string? _originalSchoolYear;
 
     public List<string> TypeOptions { get; } = ["Klasse", "Kurs"];
     [ObservableProperty] private string _selectedTypeName = "Kurs";
@@ -625,9 +625,9 @@ public partial class AddGroupDialogViewModel : ObservableObject
     public string SaveButtonText => _editingGroup is null ? "Anlegen" : "Speichern";
 
     public AddGroupDialogViewModel(IGroupRepository groups, ISubjectRepository subjects,
-        SchoolYearService sy, IEnrollmentRepository enrollments)
+        SchoolYearService sy)
     {
-        _groups = groups; _subjects = subjects; _sy = sy; _enrollments = enrollments;
+        _groups = groups; _subjects = subjects;
         _allSubjects      = subjects.GetAll();
         KnownSubjectNames = _allSubjects.Select(s => s.Name).ToList();
         SchoolYears       = sy.RecentSchoolYears(3);
@@ -642,10 +642,9 @@ public partial class AddGroupDialogViewModel : ObservableObject
     public void LoadForEdit(LearningGroup group)
     {
         _editingGroup = group;
-        _originalSchoolYear = group.SchoolYear;
         SelectedTypeName = group.Type == GroupType.Class ? "Klasse" : "Kurs";
         Name = group.Name;
-        Subject = group.Subject ?? "";
+        Subject = group.SubjectId is Guid subjectId ? _subjects.GetById(subjectId)?.Name ?? "" : "";
         GradeLevel = group.GradeLevel;
         SelectedGradingName = group.GradingSystem == GradingSystem.Grades1To6
             ? "Noten 1–6" : "Punkte 0–15";
@@ -684,7 +683,6 @@ public partial class AddGroupDialogViewModel : ObservableObject
 
         Result = _editingGroup ?? new LearningGroup();
         Result.Name          = Name.Trim();
-        Result.Subject       = subjectName;
         Result.SubjectId     = subjectId;
         Result.Type          = IsKurs ? GroupType.Course : GroupType.Class;
         Result.GradeLevel    = GradeLevel;
@@ -695,14 +693,5 @@ public partial class AddGroupDialogViewModel : ObservableObject
         Result.IsOwnClass    = IsOwnClass;
         Result.IsDifferentiated = IsDifferentiated;
         _groups.Save(Result);
-
-        if (_editingGroup is not null && _originalSchoolYear != SelectedSchoolYear)
-        {
-            foreach (var enrollment in _enrollments.GetByGroup(Result.Id))
-            {
-                enrollment.SchoolYear = SelectedSchoolYear;
-                _enrollments.Save(enrollment);
-            }
-        }
     }
 }

LehrerApp.Desktop/ViewModels/Groups/ParticipationGradeViewModels.cs

@@ -16,7 +16,7 @@ public partial class ParticipationGradeDialogViewModel : ObservableObject
     private readonly IParticipationRepository _entries;
     private readonly IParticipationAspectRepository _aspects;
     private readonly IStudentRepository _students;
-    private readonly IEnrollmentRepository _enrollments;
+    private readonly IGroupMembershipRepository _memberships;
     private readonly IGradeRepository _grades;
     private readonly GradingService _grading;
     private readonly Guid _groupId;
@@ -39,11 +39,11 @@ public partial class ParticipationGradeDialogViewModel : ObservableObject
     public ParticipationGradeDialogViewModel(
         IParticipationSessionRepository sessions, IParticipationRepository entries,
         IParticipationAspectRepository aspects, IStudentRepository students,
-        IEnrollmentRepository enrollments, IGradeRepository grades, GradingService grading,
+        IGroupMembershipRepository memberships, IGradeRepository grades, GradingService grading,
         Guid groupId, string schoolYear, GradingSystem gradingSystem)
     {
         _sessions = sessions; _entries = entries; _aspects = aspects;
-        _students = students; _enrollments = enrollments; _grades = grades;
+        _students = students; _memberships = memberships; _grades = grades;
         _grading = grading; _groupId = groupId; _schoolYear = schoolYear;
         _gradingSystem = gradingSystem;
 
@@ -83,14 +83,14 @@ public partial class ParticipationGradeDialogViewModel : ObservableObject
             .OrderBy(s => s.Date)
             .ToList();
 
-        var studentList = _students.GetByGroup(_groupId, _schoolYear);
-        var enrollmentList = _enrollments.GetByGroupAndYear(_groupId, _schoolYear);
+        var studentList = _students.GetByGroup(_groupId);
+        var membershipList = _memberships.GetByGroup(_groupId);
 
         foreach (var student in studentList.OrderBy(s => s.LastName).ThenBy(s => s.FirstName))
         {
-            var enrollment = enrollmentList.FirstOrDefault(e => e.StudentId == student.Id);
+            var membership = membershipList.FirstOrDefault(e => e.StudentId == student.Id);
             var relevantSessions = sessions
-                .Where(s => enrollment is null || IsEnrolledAtDate(enrollment, s.Date))
+                .Where(s => membership is null || IsMemberAtDate(membership, s.Date))
                 .ToList();
 
             var points = new List<(DateOnly Date, double Rating)>();
@@ -132,7 +132,6 @@ public partial class ParticipationGradeDialogViewModel : ObservableObject
                 {
                     StudentId = row.StudentId,
                     GroupId = _groupId,
-                    SchoolYear = _schoolYear,
                     Category = GradeCategory.Participation,
                     Note = noteTag,
                 };
@@ -153,12 +152,12 @@ public partial class ParticipationGradeDialogViewModel : ObservableObject
         _                      => true,
     };
 
-    private static bool IsEnrolledAtDate(Enrollment e, DateOnly date) => e.Period switch
+    private static bool IsMemberAtDate(GroupMembership membership, DateOnly date) => membership.Period switch
     {
-        EnrollmentPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
-        EnrollmentPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
-        EnrollmentPeriod.Custom => (e.JoinedAt is null || date >= e.JoinedAt.Value)
-                                 && (e.LeftAt  is null || date <= e.LeftAt.Value),
+        MembershipPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
+        MembershipPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
+        MembershipPeriod.Custom => (membership.JoinedAt is null || date >= membership.JoinedAt.Value)
+                                && (membership.LeftAt  is null || date <= membership.LeftAt.Value),
         _                       => true,
     };
 }

LehrerApp.Desktop/ViewModels/Groups/ParticipationViewModels.cs

@@ -15,7 +15,7 @@ public partial class ParticipationTabViewModel : ObservableObject
     private readonly IParticipationRepository        _entries;
     private readonly IParticipationAspectRepository  _aspects;
     private readonly IStudentRepository              _students;
-    private readonly IEnrollmentRepository           _enrollments;
+    private readonly IGroupMembershipRepository      _memberships;
     private readonly IGroupRepository                _groups;
     private readonly ICompetencyDomainRepository     _competencyDomains;
 
@@ -53,13 +53,13 @@ public partial class ParticipationTabViewModel : ObservableObject
         IParticipationRepository entries,
         IParticipationAspectRepository aspects,
         IStudentRepository students,
-        IEnrollmentRepository enrollments,
+        IGroupMembershipRepository memberships,
         IGroupRepository groups,
         ICompetencyDomainRepository competencyDomains)
     {
         _sessions         = sessions;    _entries     = entries;
         _aspects          = aspects;     _students    = students;
-        _enrollments      = enrollments; _groups      = groups;
+        _memberships      = memberships; _groups      = groups;
         _competencyDomains = competencyDomains;
     }
 
@@ -128,18 +128,18 @@ public partial class ParticipationTabViewModel : ObservableObject
         StudentRows.Clear();
         var session     = _sessions.GetById(sessionId);
         var sessionDate = session?.Date ?? DateOnly.FromDateTime(DateTime.Today);
-        var students    = _students.GetByGroup(_groupId, _schoolYear);
-        var enrollments = _enrollments.GetByGroupAndYear(_groupId, _schoolYear);
+        var students    = _students.GetByGroup(_groupId);
+        var memberships = _memberships.GetByGroup(_groupId);
         var entries     = _entries.GetBySession(sessionId);
 
         foreach (var s in students)
         {
-            var enrollment = enrollments.FirstOrDefault(e => e.StudentId == s.Id);
-            if (enrollment is not null && !IsEnrolledAtDate(enrollment, sessionDate))
+            var membership = memberships.FirstOrDefault(e => e.StudentId == s.Id);
+            if (membership is not null && !IsMemberAtDate(membership, sessionDate))
                 continue;
 
             var entry = entries.FirstOrDefault(e => e.StudentId == s.Id)
-                        ?? new ParticipationEntry { SessionId = sessionId, GroupId = _groupId, StudentId = s.Id };
+                        ?? new ParticipationEntry { SessionId = sessionId, StudentId = s.Id };
             var row = new ParticipationStudentRow(s.Id, s.FullName, entry, Aspects.ToList(), ActiveCompetencyCodes);
             row.OnRatingChanged           = (sid, key, val)  => SaveRating(sessionId, sid, key, val);
             row.OnCompetencyRatingChanged = (sid, code, val) => SaveCompetencyRating(sessionId, sid, code, val);
@@ -149,25 +149,22 @@ public partial class ParticipationTabViewModel : ObservableObject
         RebuildColumnsSignal++;
     }
 
-    private static bool IsEnrolledAtDate(Enrollment e, DateOnly date) => e.Period switch
+    private static bool IsMemberAtDate(GroupMembership membership, DateOnly date) => membership.Period switch
     {
-        EnrollmentPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
-        EnrollmentPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
-        EnrollmentPeriod.Custom => (e.JoinedAt is null || date >= e.JoinedAt.Value)
-                                 && (e.LeftAt  is null || date <= e.LeftAt.Value),
+        MembershipPeriod.H1Only => date.Month >= 8 || date.Month <= 1,
+        MembershipPeriod.H2Only => date.Month >= 2 && date.Month <= 7,
+        MembershipPeriod.Custom => (membership.JoinedAt is null || date >= membership.JoinedAt.Value)
+                                && (membership.LeftAt  is null || date <= membership.LeftAt.Value),
         _                       => true,
     };
 
     private void SaveRating(Guid sessionId, Guid studentId, string key, int? value)
     {
-        var session = _sessions.GetById(sessionId);
-        var entry   = _entries.GetBySessionAndStudent(sessionId, studentId)
+        var entry = _entries.GetBySessionAndStudent(sessionId, studentId)
                       ?? new ParticipationEntry
                       {
                           SessionId = sessionId,
-                          GroupId   = _groupId,
                           StudentId = studentId,
-                          Date      = session?.Date ?? DateOnly.FromDateTime(DateTime.Today),
                       };
 
         var existing = entry.Ratings.FirstOrDefault(r => r.Key == key);
@@ -280,14 +277,11 @@ public partial class ParticipationTabViewModel : ObservableObject
 
     private void SaveCompetencyRating(Guid sessionId, Guid studentId, string code, int? value)
     {
-        var session = _sessions.GetById(sessionId);
-        var entry   = _entries.GetBySessionAndStudent(sessionId, studentId)
+        var entry = _entries.GetBySessionAndStudent(sessionId, studentId)
                       ?? new ParticipationEntry
                       {
                           SessionId = sessionId,
-                          GroupId   = _groupId,
                           StudentId = studentId,
-                          Date      = session?.Date ?? DateOnly.FromDateTime(DateTime.Today),
                       };
         var existing = entry.CompetencyRatings.FirstOrDefault(r => r.Code == code);
         if (value is null)

LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs

@@ -110,7 +110,15 @@ public partial class SettingsViewModel : ObservableObject
     private void AddSubject()
     {
         if (string.IsNullOrWhiteSpace(NewName)) { ValidationMessage = "Name erforderlich."; return; }
-        _subjects.Save(new Subject { Name = NewName.Trim(), ShortName = NewShort.Trim() });
+        try
+        {
+            _subjects.Save(new Subject { Name = NewName.Trim(), ShortName = NewShort.Trim() });
+        }
+        catch (InvalidOperationException ex)
+        {
+            ValidationMessage = ex.Message;
+            return;
+        }
         NewName = ""; NewShort = ""; ValidationMessage = "";
         LoadSubjects();
     }
@@ -119,7 +127,16 @@ public partial class SettingsViewModel : ObservableObject
     private void DeleteSubject(SubjectListItem? item)
     {
         if (item is null) return;
-        _subjects.Delete(item.Id);
+        try
+        {
+            _subjects.Delete(item.Id);
+        }
+        catch (InvalidOperationException ex)
+        {
+            ValidationMessage = ex.Message;
+            return;
+        }
+        ValidationMessage = "";
         if (CatalogSubject?.Id == item.Id) CatalogSubject = null;
         LoadSubjects();
     }

LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs

@@ -68,8 +68,9 @@ public class StudentListItem
 public partial class StudentDetailViewModel : ObservableObject
 {
     private readonly IStudentRepository _students;
-    private readonly IEnrollmentRepository _enrollments;
+    private readonly IGroupMembershipRepository _memberships;
     private readonly IGroupRepository _groups;
+    private readonly ISubjectRepository _subjects;
     private readonly IDocumentationRepository _docs;
 
     [ObservableProperty] private Student? _student;
@@ -79,7 +80,7 @@ public partial class StudentDetailViewModel : ObservableObject
     [ObservableProperty] private string _editLastName = "";
     [ObservableProperty] private ContactItem? _selectedContact;
 
-    public ObservableCollection<EnrollmentEntry> Enrollments   { get; } = [];
+    public ObservableCollection<GroupMembershipEntry> GroupMemberships { get; } = [];
     public ObservableCollection<DocEntry>        Documentation { get; } = [];
     public ObservableCollection<ContactItem>     Contacts      { get; } = [];
     public bool HasNoContacts => Contacts.Count == 0;
@@ -87,11 +88,11 @@ public partial class StudentDetailViewModel : ObservableObject
     public Action<ContactItem>? OnViewAddress { get; set; }
 
     public StudentDetailViewModel(IStudentRepository students,
-        IEnrollmentRepository enrollments, IGroupRepository groups,
+        IGroupMembershipRepository memberships, IGroupRepository groups, ISubjectRepository subjects,
         IDocumentationRepository docs)
     {
-        _students = students; _enrollments = enrollments;
-        _groups = groups; _docs = docs;
+        _students = students; _memberships = memberships;
+        _groups = groups; _subjects = subjects; _docs = docs;
     }
 
     public void LoadStudent(Guid id)
@@ -102,12 +103,13 @@ public partial class StudentDetailViewModel : ObservableObject
         EditFirstName = Student.FirstName;
         EditLastName  = Student.LastName;
 
-        Enrollments.Clear();
-        foreach (var e in _enrollments.GetByStudent(Student.Id))
+        GroupMemberships.Clear();
+        foreach (var membership in _memberships.GetByStudent(Student.Id))
         {
-            var g = _groups.GetById(e.GroupId);
+            var g = _groups.GetById(membership.GroupId);
             if (g is null) continue;
-            Enrollments.Add(new() { SchoolYear = e.SchoolYear, GroupName = g.Name, Subject = g.Subject ?? "" });
+            var subject = g.SubjectId is Guid subjectId ? _subjects.GetById(subjectId)?.Name ?? "" : "";
+            GroupMemberships.Add(new() { SchoolYear = g.SchoolYear, GroupName = g.Name, Subject = subject });
         }
 
         LoadContacts();
@@ -195,7 +197,7 @@ public partial class StudentDetailViewModel : ObservableObject
     private bool CanViewSelectedAddress() => SelectedContact?.HasAddress == true;
 }
 
-public class EnrollmentEntry { public string SchoolYear { get; set; } = ""; public string GroupName { get; set; } = ""; public string Subject { get; set; } = ""; }
+public class GroupMembershipEntry { public string SchoolYear { get; set; } = ""; public string GroupName { get; set; } = ""; public string Subject { get; set; } = ""; }
 public class DocEntry { public string Date { get; set; } = ""; public string Title { get; set; } = ""; public string TypeLabel { get; set; } = ""; public bool IsConfidential { get; set; } }
 
 public class ContactItem

LehrerApp.Desktop/Views/Groups/GroupDetailView.axaml.cs

@@ -32,9 +32,8 @@ public partial class GroupDetailView : UserControl
 
         var dialogVm = new AddStudentToGroupDialogViewModel(
             App.Services.GetRequiredService<IStudentRepository>(),
-            App.Services.GetRequiredService<IEnrollmentRepository>(),
-            vm.Group.Id,
-            vm.Group.SchoolYear);
+            App.Services.GetRequiredService<IGroupMembershipRepository>(),
+            vm.Group.Id);
 
         var dialog = new AddStudentToGroupDialog { DataContext = dialogVm };
         var owner = TopLevel.GetTopLevel(this) as Window;
@@ -62,7 +61,7 @@ public partial class GroupDetailView : UserControl
             App.Services.GetRequiredService<IGradingKeyTemplateRepository>(),
             App.Services.GetRequiredService<GradingService>(),
             groupId, vm.Group.SubjectId, vm.Group.GradeLevel, vm.Group.GradingSystem,
-            vm.Group.Subject ?? "", vm.Group.IsDifferentiated, editingExam, duplicateSource);
+            vm.SubjectName, vm.Group.IsDifferentiated, editingExam, duplicateSource);
 
         var dialog = new ExamDialog { DataContext = dialogVm };
         var owner = TopLevel.GetTopLevel(this) as Window;
@@ -85,9 +84,9 @@ public partial class GroupDetailView : UserControl
         var dialogVm = new ExamGradingDialogViewModel(
             App.Services.GetRequiredService<IExamResultRepository>(),
             App.Services.GetRequiredService<IStudentRepository>(),
-            App.Services.GetRequiredService<IEnrollmentRepository>(),
+            App.Services.GetRequiredService<IGroupMembershipRepository>(),
             App.Services.GetRequiredService<GradingService>(),
-            exam, vm.Group.Id, vm.Group.SchoolYear);
+            exam, vm.Group.Id);
 
         var dialog = new ExamGradingDialog { DataContext = dialogVm };
         var owner = TopLevel.GetTopLevel(this) as Window;

LehrerApp.Desktop/Views/Groups/ParticipationTabView.axaml.cs

@@ -153,7 +153,7 @@ public partial class ParticipationTabView : UserControl
             App.Services.GetRequiredService<IParticipationRepository>(),
             App.Services.GetRequiredService<IParticipationAspectRepository>(),
             App.Services.GetRequiredService<IStudentRepository>(),
-            App.Services.GetRequiredService<IEnrollmentRepository>(),
+            App.Services.GetRequiredService<IGroupMembershipRepository>(),
             App.Services.GetRequiredService<IGradeRepository>(),
             App.Services.GetRequiredService<GradingService>(),
             tabVm.GroupId, tabVm.SchoolYear, tabVm.GradingSystem);

LehrerApp.Desktop/Views/Students/StudentDetailView.axaml

@@ -40,9 +40,9 @@
         <ScrollViewer Padding="20">
           <StackPanel Spacing="12">
             <TextBlock Text="Lerngruppen" FontSize="15" FontWeight="SemiBold"/>
-            <ItemsControl ItemsSource="{Binding Enrollments}">
+            <ItemsControl ItemsSource="{Binding GroupMemberships}">
               <ItemsControl.ItemTemplate>
-                <DataTemplate DataType="vm:EnrollmentEntry">
+                <DataTemplate DataType="vm:GroupMembershipEntry">
                   <Border Background="{DynamicResource SystemControlBackgroundAltHighBrush}"
                           CornerRadius="6" Padding="12,8" Margin="0,0,0,6">
                     <Grid ColumnDefinitions="70,*,Auto">
@@ -54,8 +54,8 @@
                 </DataTemplate>
               </ItemsControl.ItemTemplate>
             </ItemsControl>
-            <TextBlock Text="Keine Einschreibungen vorhanden." Opacity="0.4"
-                       IsVisible="{Binding !Enrollments.Count}"/>
+            <TextBlock Text="Keine Lerngruppenzuordnungen vorhanden." Opacity="0.4"
+                       IsVisible="{Binding !GroupMemberships.Count}"/>
           </StackPanel>
         </ScrollViewer>
       </ContentPage>

LehrerApp.sln

@@ -10,6 +10,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "LehrerApp.Api", "LehrerApp.
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "LehrerApp.Desktop", "LehrerApp.Desktop\LehrerApp.Desktop.csproj", "{A1000005-0000-0000-0000-000000000005}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "LehrerApp.Data.Tests", "LehrerApp.Data.Tests\LehrerApp.Data.Tests.csproj", "{A1000006-0000-0000-0000-000000000006}"
+EndProject
 Global
   GlobalSection(SolutionConfigurationPlatforms) = preSolution
     Debug|Any CPU = Debug|Any CPU
@@ -26,5 +28,7 @@ Global
     {A1000004-0000-0000-0000-000000000004}.Debug|Any CPU.Build.0 = Debug|Any CPU
     {A1000005-0000-0000-0000-000000000005}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
     {A1000005-0000-0000-0000-000000000005}.Debug|Any CPU.Build.0 = Debug|Any CPU
+    {A1000006-0000-0000-0000-000000000006}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+    {A1000006-0000-0000-0000-000000000006}.Debug|Any CPU.Build.0 = Debug|Any CPU
   EndGlobalSection
 EndGlobal

TODO.md

@@ -33,7 +33,7 @@ Modelle `Exam`, `ExamTask`, `GradingKeyEntry`, `ExamResult` existieren bereits i
 
 **Niveaudifferenzierung (E/G/Förder) ist bereits umgesetzt** — nicht aus dieser Liste, sondern
 ein separater Bedarf (Nachteilsausgleich/Binnendifferenzierung): `LearningGroup.IsDifferentiated`
-(Checkbox in den Stammdaten) blendet eine Niveau-Zuordnung je Schüler ein (`Enrollment.Niveau`,
+(Checkbox in den Stammdaten) blendet eine Niveau-Zuordnung je Schüler ein (`GroupMembership.Niveau`,
 Schüler-Tab). Klausuren bekommen optional ein `Niveau`; Punkteeingabe und Auswertung filtern dann
 automatisch auf die passenden Schüler. Workflow: G-Klausur normal anlegen, per "Duplizieren"
 (1.1.4) die E-Variante mit eigenen Aufgaben/Punkten ableiten (Niveau wird beim Duplizieren bewusst
@@ -109,7 +109,7 @@ Der Tab "Noten" in [GroupDetailView.axaml:83](LehrerApp.Desktop/Views/Groups/Gro
       sonstige Noten). Zelle zeigt Note/Punkte.
 - [ ] **2.1.2** Spalte "Gesamt" mit gewichtetem Durchschnitt über `GradingService.WeightedAverage()`.
 - [ ] **2.1.3** Sortierung nach Name / Gesamtnote, Umschalten Noten ↔ Punkte.
-- [ ] **2.1.4** Halbjahresfilter (H1 / H2 / Gesamtjahr), berücksichtigt `Enrollment.Period`.
+- [ ] **2.1.4** Halbjahresfilter (H1 / H2 / Gesamtjahr), berücksichtigt `GroupMembership.Period`.
 
 ### 2.2 Einzelnoten pflegen
 - [ ] **2.2.1** Dialog "Note hinzufügen": Kategorie (`GradeCategory`), Wert, Datum, Gewichtung, Notiz.
@@ -276,7 +276,7 @@ Navigationspunkt "Arbeitszeit" ist ein `PlaceholderViewModel`.
 
 Grundfunktionen sind vorhanden ([StudentViewModels.cs](LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs),
 [GroupViewModels.cs](LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs)).
-Niveau-Zuordnung je Schüler (E/G/Förder, `Enrollment.Niveau`) ist bereits umgesetzt, siehe
+Niveau-Zuordnung je Schüler (E/G/Förder, `GroupMembership.Niveau`) ist bereits umgesetzt, siehe
 Hinweis in Kapitel 1 — betrifft auch Kurse, nicht nur Klassen.
 
 ### 7.1 Schüler
@@ -291,9 +291,9 @@ Hinweis in Kapitel 1 — betrifft auch Kurse, nicht nur Klassen.
       `ToggleArchiveCommand` in `GroupListViewModel`), nicht Teil der aktuellen Klausuren-Arbeit,
       beim Review aber bestätigt.
 - [ ] **7.2.2** Gruppe ins neue Schuljahr übernehmen: Kopie mit gleicher Schülerschaft,
-      neues `SchoolYear`, neue `Enrollment`-Einträge.
-- [ ] **7.2.3** Schüler aus einer Gruppe entfernen (`Enrollment.LeftAt` setzen statt löschen).
-- [ ] **7.2.4** Umgang mit `EnrollmentPeriod.Custom` in allen Auswertungen prüfen
+      neues `SchoolYear`, neue `GroupMembership`-Einträge.
+- [ ] **7.2.3** Schüler aus einer Gruppe entfernen (`GroupMembership.LeftAt` setzen statt löschen).
+- [ ] **7.2.4** Umgang mit `MembershipPeriod.Custom` in allen Auswertungen prüfen
       (Schüler zählt nur im belegten Zeitraum).
 - [ ] **7.2.5** Archivansicht abgeschlossener Schuljahre (schreibgeschützt).
 

docs/Datenmodell.md

@@ -0,0 +1,73 @@
+# Datenmodell und Begriffe
+
+Dieses Dokument beschreibt die fachliche Bedeutung der zentralen Datensätze.
+Es soll verhindern, dass technisch ähnliche Felder als unterschiedliche
+Sachverhalte interpretiert oder dieselben Informationen mehrfach gespeichert
+werden.
+
+## Lerngruppe (`LearningGroup`)
+
+Eine Lerngruppe ist die konkrete Unterrichtsgruppe eines Fachs in genau einem
+Schuljahr. Sie kann als Klasse oder Kurs organisiert sein.
+
+- `Name`: frei gewählte Bezeichnung, zum Beispiel `5b NAT (BEN)` oder `Mathe G`
+- `SubjectId`: Verweis auf das unterrichtete Fach
+- `SchoolYear`: Schuljahr dieser konkreten Lerngruppe
+- `GradeLevel`: Klassen- beziehungsweise Jahrgangsstufe
+
+Der Fachname wird ausschließlich im `Subject`-Stammdatensatz gepflegt. Die
+Lerngruppe speichert keine zweite Kopie des Fachnamens.
+
+## Gruppenzuordnung (`GroupMembership`)
+
+Eine Gruppenzuordnung verbindet einen Schüler mit einer Lerngruppe. Sie ist
+keine Aufnahme oder Einschreibung an der Schule.
+
+- `StudentId`: Schüler
+- `GroupId`: Lerngruppe
+- `AddedOn`: Tag, an dem die Zuordnung in der App angelegt wurde
+- `Period`: ganzes Schuljahr, erstes Halbjahr, zweites Halbjahr oder eigener Zeitraum
+- `JoinedAt` / `LeftAt`: Grenzen eines eigenen Teilnahmezeitraums
+- `Niveau`: optionale Niveaudifferenzierung
+
+Das Schuljahr wird über die Lerngruppe ermittelt und deshalb nicht zusätzlich
+in der Gruppenzuordnung gespeichert. Für eine spätere echte Schulaufnahme wäre
+ein eigenes Feld wie `Student.SchoolEntryDate` zu verwenden.
+
+Pro Kombination aus Schüler und Lerngruppe darf es höchstens eine Zuordnung
+geben.
+
+## Bewusst gespeicherte Momentaufnahmen
+
+Einige berechnete Werte bleiben absichtlich gespeichert:
+
+- `ExamResult.TotalPoints` und `ExamResult.Grade` halten das zuletzt berechnete
+  Klausurergebnis fest.
+- `Exam.GradingKey` hält den für die konkrete Klausur verwendeten Notenschlüssel
+  fest und ist unabhängig von später geänderten Vorlagen.
+- `Exam.Tasks` hält die Aufgabenstruktur der konkreten Klausur fest.
+
+Änderungen an Aufgaben oder Notenschlüssel müssen die betroffenen Ergebnisse
+kontrolliert neu berechnen. Diese Werte sind daher fachliche Momentaufnahmen und
+nicht bloß unkontrollierte Kopien.
+
+## Bewusste Denormalisierung
+
+`Lesson.GroupId` bleibt zusätzlich zu `Lesson.UnitId` gespeichert. Dadurch kann
+der häufige Kalenderzugriff auf alle Stunden einer Lerngruppe direkt indiziert
+werden. Beim späteren Ausbau der Unterrichtsplanung muss sichergestellt werden,
+dass `Lesson.GroupId` mit der Lerngruppe der zugehörigen Einheit übereinstimmt.
+
+## Eindeutige Schlüssel
+
+Die Datenbank schützt folgende Kombinationen mit eindeutigen Indizes:
+
+- Gruppenzuordnung: `StudentId + GroupId`
+- Klausurergebnis: `ExamId + StudentId`
+- Mitarbeitseintrag: `SessionId + StudentId`
+- Fach: normalisierter Fachname
+
+Altdaten werden beim Öffnen der Datenbank automatisch migriert. Die Migration
+verknüpft bisherige Fachtexte mit den Fachstammdaten, benennt die bisherige
+`enrollments`-Collection in `group_memberships` um und entfernt daraus das
+doppelt gespeicherte Schuljahr.