Record latest migration ID in model snapshot as property to detect diverged migration trees

src/EFCore.Design/Migrations/Design/CSharpMigrationsGenerator.cs

@@ -240,12 +240,14 @@ public override string GenerateMetadata(
     /// <param name="contextType">The model snapshot's <see cref="DbContext" /> type.</param>
     /// <param name="modelSnapshotName">The model snapshot's name.</param>
     /// <param name="model">The model.</param>
+    /// <param name="latestMigrationId">The ID of the latest migration that has been applied to the model.</param>
     /// <returns>The model snapshot code.</returns>
     public override string GenerateSnapshot(
         string? modelSnapshotNamespace,
         Type contextType,
         string modelSnapshotName,
-        IModel model)
+        IModel model,
+        string? latestMigrationId = null)
     {
         var builder = new IndentedStringBuilder();
         AppendAutoGeneratedTag(builder);
@@ -288,6 +290,16 @@ public override string GenerateSnapshot(
             .AppendLine("{");
         using (builder.Indent())
         {
+            if (!string.IsNullOrEmpty(latestMigrationId))
+            {
+                builder
+                    .AppendLine("// If you encounter a merge conflict in the line below, it means you need to")
+                    .AppendLine("// discard one of the migration branches and recreate its migrations on top of")
+                    .AppendLine("// the other branch. See https://aka.ms/efcore-docs-migrations-conflicts for more info.")
+                    .Append("public override string LatestMigrationId => ").Append(Code.Literal(latestMigrationId)).AppendLine(";")
+                    .AppendLine();
+            }
+
             builder
                 .AppendLine("protected override void BuildModel(ModelBuilder modelBuilder)")
                 .AppendLine("{")

src/EFCore.Design/Migrations/Design/IMigrationsCodeGenerator.cs

@@ -49,12 +49,14 @@ string GenerateMigration(
     /// <param name="contextType">The model snapshot's <see cref="DbContext" /> type.</param>
     /// <param name="modelSnapshotName">The model snapshot's name.</param>
     /// <param name="model">The model.</param>
+    /// <param name="latestMigrationId">The ID of the latest migration that has been applied to the model.</param>
     /// <returns>The model snapshot code.</returns>
     string GenerateSnapshot(
         string? modelSnapshotNamespace,
         Type contextType,
         string modelSnapshotName,
-        IModel model);
+        IModel model,
+        string? latestMigrationId = null);
 
     /// <summary>
     ///     Gets the file extension code files should use.

src/EFCore.Design/Migrations/Design/MigrationsCodeGenerator.cs

@@ -76,12 +76,14 @@ public abstract string GenerateMetadata(
     /// <param name="contextType">The model snapshot's <see cref="DbContext" /> type.</param>
     /// <param name="modelSnapshotName">The model snapshot's name.</param>
     /// <param name="model">The model.</param>
+    /// <param name="latestMigrationId">The ID of the latest migration that has been applied to the model.</param>
     /// <returns>The model snapshot code.</returns>
     public abstract string GenerateSnapshot(
         string? modelSnapshotNamespace,
         Type contextType,
         string modelSnapshotName,
-        IModel model);
+        IModel model,
+        string? latestMigrationId = null);
 
     /// <summary>
     ///     Gets the namespaces required for a list of <see cref="MigrationOperation" /> objects.

src/EFCore.Design/Migrations/Design/MigrationsScaffolder.cs

@@ -187,7 +187,8 @@ public virtual ScaffoldedMigration ScaffoldMigration(
             modelSnapshotNamespace,
             _contextType,
             modelSnapshotName,
-            Dependencies.Model);
+            Dependencies.Model,
+            migrationId);
 
         return new ScaffoldedMigration(
             codeGenerator.FileExtension,
@@ -346,6 +347,8 @@ public virtual MigrationFiles RemoveMigration(
             }
         }
 
+        var latestMigrationId = migrations.Count > 1 ? migrations[^2].GetId() : null;
+
         var modelSnapshotName = modelSnapshot.GetType().Name;
         var modelSnapshotFileName = modelSnapshotName + codeGenerator.FileExtension;
         var modelSnapshotFile = TryGetProjectFile(projectDir, modelSnapshotFileName);
@@ -378,7 +381,8 @@ public virtual MigrationFiles RemoveMigration(
                 modelSnapshotNamespace,
                 _contextType,
                 modelSnapshotName,
-                model);
+                model,
+                latestMigrationId);
 
             modelSnapshotFile ??= Path.Combine(
                 GetDirectory(projectDir, null, GetSubNamespace(rootNamespace, modelSnapshotNamespace)),

src/EFCore.Relational/Infrastructure/ModelSnapshot.cs

@@ -28,6 +28,15 @@ private IModel CreateModel()
     public virtual IModel Model
         => _model ??= CreateModel();
 
+    /// <summary>
+    ///     The ID of the latest migration applied to the model when the snapshot was created.
+    /// </summary>
+    /// <remarks>
+    ///     See <see href="https://aka.ms/efcore-docs-migrations-conflicts">Database migrations</see> for more information and examples.
+    /// </remarks>
+    public virtual string? LatestMigrationId
+        => null;
+
     /// <summary>
     ///     Called lazily by <see cref="Model" /> to build the model snapshot
     ///     the first time it is requested.

test/EFCore.Design.Tests/Design/OperationExecutorTest.cs

@@ -180,6 +180,11 @@ namespace My.Gnomespace.Data
     [DbContext(typeof(OperationExecutorTest.GnomeContext))]
     partial class GnomeContextModelSnapshot : ModelSnapshot
     {
+        // If you encounter a merge conflict in the line below, it means you need to
+        // discard one of the migration branches and recreate its migrations on top of
+        // the other branch. See https://aka.ms/efcore-docs-migrations-conflicts for more info.
+        public override string LatestMigrationId => "11112233445566_{{migrationName}}";
+
         protected override void BuildModel(ModelBuilder modelBuilder)
         {
 #pragma warning disable 612, 618

test/EFCore.Design.Tests/Migrations/Design/CSharpMigrationsGeneratorTest.ModelSnapshot.cs

@@ -122,6 +122,63 @@ protected override void BuildModel(ModelBuilder modelBuilder)
         Assert.Equal(2, snapshot.Model.GetEntityTypes().Count());
     }
 
+    [ConditionalFact]
+    public void Snapshot_with_migration_id()
+    {
+        var generator = CreateMigrationsCodeGenerator();
+
+        var modelBuilder = FakeRelationalTestHelpers.Instance.CreateConventionBuilder();
+        modelBuilder.Entity<EntityWithConstructorBinding>(x =>
+        {
+            x.Property(e => e.Id);
+        });
+
+        var finalizedModel = modelBuilder.FinalizeModel(designTime: true);
+
+        var modelSnapshotCode = generator.GenerateSnapshot(
+            "MyNamespace",
+            typeof(MyContext),
+            "MySnapshot",
+            finalizedModel,
+            "20240101120000_InitialCreate");
+
+        Assert.Contains("public override string LatestMigrationId => \"20240101120000_InitialCreate\";", modelSnapshotCode);
+        Assert.Contains("// If you encounter a merge conflict in the line below, it means you need to", modelSnapshotCode);
+        Assert.Contains("// discard one of the migration branches and recreate its migrations on top of", modelSnapshotCode);
+        Assert.Contains("// the other branch. See https://aka.ms/efcore-docs-migrations-conflicts for more info.", modelSnapshotCode);
+
+        var snapshot = CompileModelSnapshot(modelSnapshotCode, "MyNamespace.MySnapshot", typeof(MyContext));
+        Assert.NotNull(snapshot.Model);
+        Assert.Equal("20240101120000_InitialCreate", snapshot.LatestMigrationId);
+    }
+
+    [ConditionalFact]
+    public void Snapshot_without_migration_id()
+    {
+        var generator = CreateMigrationsCodeGenerator();
+
+        var modelBuilder = FakeRelationalTestHelpers.Instance.CreateConventionBuilder();
+        modelBuilder.Entity<EntityWithConstructorBinding>(x =>
+        {
+            x.Property(e => e.Id);
+        });
+
+        var finalizedModel = modelBuilder.FinalizeModel(designTime: true);
+
+        var modelSnapshotCode = generator.GenerateSnapshot(
+            "MyNamespace",
+            typeof(MyContext),
+            "MySnapshot",
+            finalizedModel);
+
+        Assert.DoesNotContain("LatestMigrationId", modelSnapshotCode);
+        Assert.DoesNotContain("merge conflict", modelSnapshotCode);
+
+        var snapshot = CompileModelSnapshot(modelSnapshotCode, "MyNamespace.MySnapshot", typeof(MyContext));
+        Assert.NotNull(snapshot.Model);
+        Assert.Null(snapshot.LatestMigrationId);
+    }
+
     [ConditionalFact]
     public void Snapshot_default_values_are_round_tripped()
     {

test/EFCore.SqlServer.HierarchyId.Tests/MigrationTests.cs

@@ -15,7 +15,7 @@ public class MigrationTests
 {
     private delegate string MigrationCodeGetter(string migrationName, string rootNamespace);
 
-    private delegate string SnapshotCodeGetter(string rootNamespace);
+    private delegate string SnapshotCodeGetter(string rootNamespace, string migrationId);
 
     [ConditionalFact]
     public void Migration_and_snapshot_generate_with_typed_array()
@@ -39,9 +39,6 @@ private static void ValidateMigrationAndSnapshotCode(
         const string migrationName = "MyMigration";
         const string rootNamespace = "MyApp.Data";
 
-        var expectedMigration = migrationCodeGetter(migrationName, rootNamespace);
-        var expectedSnapshot = snapshotCodeGetter(rootNamespace);
-
         var reporter = new OperationReporter(
             new OperationReportHandler(
                 m => Console.WriteLine($"  error: {m}"),
@@ -59,6 +56,9 @@ private static void ValidateMigrationAndSnapshotCode(
             .GetRequiredService<IMigrationsScaffolder>()
             .ScaffoldMigration(migrationName, rootNamespace);
 
+        var expectedMigration = migrationCodeGetter(migrationName, rootNamespace);
+        var expectedSnapshot = snapshotCodeGetter(rootNamespace, migration.MigrationId);
+
         Assert.Equal(expectedMigration, migration.MigrationCode, ignoreLineEndingDifferences: true);
         Assert.Equal(expectedSnapshot, migration.SnapshotCode, ignoreLineEndingDifferences: true);
     }

test/EFCore.SqlServer.HierarchyId.Tests/TestModels/Migrations/AnonymousArraySeedContext.cs

@@ -116,7 +116,7 @@ protected override void Down(MigrationBuilder migrationBuilder)
 }}
 ";
 
-    public override string GetExpectedSnapshotCode(string rootNamespace)
+    public override string GetExpectedSnapshotCode(string rootNamespace, string migrationId)
         => $@"// <auto-generated />
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
@@ -131,6 +131,11 @@ namespace {rootNamespace}.Migrations
     [DbContext(typeof({ThisType.Name}))]
     partial class {ThisType.Name}ModelSnapshot : ModelSnapshot
     {{
+        // If you encounter a merge conflict in the line below, it means you need to
+        // discard one of the migration branches and recreate its migrations on top of
+        // the other branch. See https://aka.ms/efcore-docs-migrations-conflicts for more info.
+        public override string LatestMigrationId => ""{migrationId}"";
+
         protected override void BuildModel(ModelBuilder modelBuilder)
         {{
 #pragma warning disable 612, 618

test/EFCore.SqlServer.HierarchyId.Tests/TestModels/Migrations/MigrationContext.cs

@@ -45,5 +45,5 @@ protected void RemoveVariableModelAnnotations(ModelBuilder modelBuilder)
     }
 
     public abstract string GetExpectedMigrationCode(string migrationName, string rootNamespace);
-    public abstract string GetExpectedSnapshotCode(string rootNamespace);
+    public abstract string GetExpectedSnapshotCode(string rootNamespace, string migrationId);
 }

test/EFCore.SqlServer.HierarchyId.Tests/TestModels/Migrations/TypedArraySeedContext.cs

@@ -116,7 +116,7 @@ protected override void Down(MigrationBuilder migrationBuilder)
 }}
 ";
 
-    public override string GetExpectedSnapshotCode(string rootNamespace)
+    public override string GetExpectedSnapshotCode(string rootNamespace, string migrationId)
         => $@"// <auto-generated />
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
@@ -131,6 +131,11 @@ namespace {rootNamespace}.Migrations
     [DbContext(typeof({ThisType.Name}))]
     partial class {ThisType.Name}ModelSnapshot : ModelSnapshot
     {{
+        // If you encounter a merge conflict in the line below, it means you need to
+        // discard one of the migration branches and recreate its migrations on top of
+        // the other branch. See https://aka.ms/efcore-docs-migrations-conflicts for more info.
+        public override string LatestMigrationId => ""{migrationId}"";
+
         protected override void BuildModel(ModelBuilder modelBuilder)
         {{
 #pragma warning disable 612, 618