Fix complex property JSON column not marked nullable in TPH hierarchy (#37781)

Fixes #37404

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

Author: Copilot

src/EFCore.Relational/Metadata/Internal/RelationalModel.cs

@@ -634,8 +634,13 @@ private static void CreateContainerColumn<TColumnMappingBase>(
         {
             complexType = (IComplexType)mappedType;
 #pragma warning disable EF1001 // Internal EF Core API usage.
+            var chain = complexType.ComplexProperty.GetChainToComplexProperty(fromEntity: true);
             jsonColumn.IsNullable = complexType.ComplexProperty.IsNullable
-                || complexType.ComplexProperty.GetChainToComplexProperty(fromEntity: true).Any(p => p.IsNullable);
+                || (chain[0].DeclaringType is IEntityType declaringEntityType
+                    && declaringEntityType.BaseType != null
+                    && (declaringEntityType.GetMappingStrategy() ?? RelationalAnnotationNames.TphMappingStrategy)
+                        == RelationalAnnotationNames.TphMappingStrategy)
+                || chain.Any(p => p.IsNullable);
 #pragma warning restore EF1001 // Internal EF Core API usage.
         }
     }

test/EFCore.Relational.Tests/Metadata/RelationalModelTest.cs

@@ -3201,6 +3201,26 @@ public void Container_column_type_is_used_for_complex_collection_json_column()
             Assert.IsType<JsonColumn>(jsonColumn);
         }
 
+        [ConditionalFact]
+        public void Complex_property_json_column_is_nullable_in_TPH_hierarchy()
+        {
+            var modelBuilder = CreateConventionModelBuilder();
+
+            modelBuilder.Entity<TphBaseEntity>();
+            modelBuilder.Entity<EntityWithoutComplexProperty>();
+            modelBuilder.Entity<TphEntityWithComplexProperty>()
+                .ComplexProperty(e => e.ComplexProperty, b => b.ToJson());
+
+            var model = modelBuilder.FinalizeModel();
+            var relationalModel = model.GetRelationalModel();
+
+            var table = relationalModel.Tables.Single();
+            var jsonColumn = table.Columns.Single(c => c.Name == "ComplexProperty");
+
+            Assert.True(jsonColumn.IsNullable);
+            Assert.IsType<JsonColumn>(jsonColumn);
+        }
+
         private static IRelationalModel Finalize(TestHelpers.TestModelBuilder modelBuilder)
             => modelBuilder.FinalizeModel(designTime: true).GetRelationalModel();
 
@@ -3318,6 +3338,18 @@ private class EntityWithComplexCollection
             public List<ComplexData> ComplexCollection { get; set; }
         }
 
+        private abstract class TphBaseEntity
+        {
+            public int Id { get; set; }
+        }
+
+        private class EntityWithoutComplexProperty : TphBaseEntity;
+
+        private class TphEntityWithComplexProperty : TphBaseEntity
+        {
+            public ComplexData ComplexProperty { get; set; }
+        }
+
         private class ComplexData
         {
             public string Value { get; set; }

test/EFCore.SqlServer.FunctionalTests/Scaffolding/Baselines/ComplexTypes/DbContextModelBuilder.cs

@@ -406,7 +406,10 @@ private IRelationalModel CreateRelationalModel()
             var flagsEnum2Column = new Column("FlagsEnum2", "int", principalBaseTable);
             principalBaseTable.Columns.Add("FlagsEnum2", flagsEnum2Column);
             flagsEnum2Column.Accessors = ColumnAccessorsFactory.CreateGeneric<int>(flagsEnum2Column);
-            var manyOwnedColumn = new JsonColumn("ManyOwned", "nvarchar(max)", principalBaseTable);
+            var manyOwnedColumn = new JsonColumn("ManyOwned", "nvarchar(max)", principalBaseTable)
+            {
+                IsNullable = true
+            };
             principalBaseTable.Columns.Add("ManyOwned", manyOwnedColumn);
             manyOwnedColumn.Accessors = ColumnAccessorsFactory.CreateGeneric<JsonTypePlaceholder>(manyOwnedColumn);
             var owned_NumberColumn = new Column("Owned_Number", "int", principalBaseTable);