[release/10.0] Fix migration discovery when [DbContext] is only on base class (#37437)

Fixes #37431

---------

Co-authored-by: AndriySvyryd <6539701+AndriySvyryd@users.noreply.github.com>

Author: Copilot

src/EFCore.Relational/Migrations/Internal/MigrationsAssembly.cs

@@ -43,6 +43,27 @@ public MigrationsAssembly(
         _logger = logger;
     }
 
+    private static Type? GetDbContextType(TypeInfo typeInfo)
+    {
+        // Walk the inheritance chain to find the first DbContextAttribute.
+        // This supports inheritance while avoiding AmbiguousMatchException
+        // that would occur with GetCustomAttribute(inherit: true) when multiple
+        // attributes exist in the hierarchy.
+        var currentType = typeInfo.AsType();
+        while (currentType != null && currentType != typeof(object))
+        {
+            var attribute = currentType.GetCustomAttribute<DbContextAttribute>(inherit: false);
+            if (attribute != null)
+            {
+                return attribute.ContextType;
+            }
+
+            currentType = currentType.BaseType;
+        }
+
+        return null;
+    }
+
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
@@ -60,7 +81,7 @@ IReadOnlyDictionary<string, TypeInfo> Create()
                 var items
                     = from t in Assembly.GetConstructibleTypes()
                       where t.IsSubclassOf(typeof(Migration))
-                          && t.GetCustomAttribute<DbContextAttribute>(inherit: false)?.ContextType == _contextType
+                          && GetDbContextType(t) == _contextType
                       let id = t.GetCustomAttribute<MigrationAttribute>()?.Id
                       orderby id
                       select (id, t);
@@ -94,7 +115,7 @@ public virtual ModelSnapshot? ModelSnapshot
         => _modelSnapshot
             ??= (from t in Assembly.GetConstructibleTypes()
                  where t.IsSubclassOf(typeof(ModelSnapshot))
-                     && t.GetCustomAttribute<DbContextAttribute>(inherit: false)?.ContextType == _contextType
+                     && GetDbContextType(t) == _contextType
                  select (ModelSnapshot)Activator.CreateInstance(t.AsType())!)
             .FirstOrDefault();
 

test/EFCore.Relational.Tests/Migrations/Internal/MigrationsAssemblyTest.cs

@@ -97,6 +97,47 @@ public void Migrations_handles_inherited_DbContextAttribute()
         Assert.Contains(result, t => t.Key == "20150302103200_InheritedMigration");
     }
 
+    [ConditionalFact]
+    public void Migrations_finds_attribute_on_base_class_only()
+    {
+        var assembly = CreateMigrationsAssemblyWithAttributeOnBaseOnly();
+
+        // This should find the migration even though the attribute is only on the base class
+        var result = assembly.Migrations;
+
+        Assert.Single(result);
+        Assert.Contains(result, t => t.Key == "20150302103300_DerivedMigrationWithBaseAttribute");
+    }
+
+    private IMigrationsAssembly CreateMigrationsAssemblyWithAttributeOnBaseOnly()
+        => new MigrationsAssembly(
+            new CurrentDbContext(new AttributeOnBaseContext()),
+            new DbContextOptions<DbContext>(
+                new Dictionary<Type, IDbContextOptionsExtension>
+                {
+                    { typeof(FakeRelationalOptionsExtension), new FakeRelationalOptionsExtension() }
+                }),
+            new MigrationsIdGenerator(),
+            new FakeDiagnosticsLogger<DbLoggerCategory.Migrations>());
+
+    private class AttributeOnBaseContext : DbContext;
+
+    [DbContext(typeof(AttributeOnBaseContext))]
+    private class BaseMigrationWithAttribute : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+        }
+    }
+
+    [Migration("20150302103300_DerivedMigrationWithBaseAttribute")]
+    private class DerivedMigrationWithBaseAttribute : BaseMigrationWithAttribute
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+        }
+    }
+
     private IMigrationsAssembly CreateInheritedMigrationsAssembly()
         => new MigrationsAssembly(
             new CurrentDbContext(new DerivedContext()),