fix(model): only carry scalar column defaults as server_default in migrations

news/6706.bugfix.md

@@ -0,0 +1 @@
+Fixed `reflex db makemigrations`/`migrate` crashing with `CompileError` when autogenerating a migration that adds a column with a callable default (e.g. `default_factory=datetime.now` or `default=uuid.uuid4`) to an existing table; callable defaults are now evaluated before being carried as a SQL `server_default`.

reflex/model.py

@@ -408,10 +408,14 @@ def render_add_column_with_server_default(
             op: Any,
         ):
             # Carry the sqlmodel default as server_default so that newly added
-            # columns get the desired default value in existing rows.
+            # columns get the desired default value in existing rows. Only scalar
+            # defaults can be rendered as a SQL literal. Evaluate callable defaults
+            # once so non-nullable columns can be added to tables with existing rows.
             if op.column.default is not None and op.column.server_default is None:
+                default = op.column.default
+                value = default.arg(None) if default.is_callable else default.arg
                 op.column.server_default = sqlalchemy.DefaultClause(
-                    sqlalchemy.sql.expression.literal(op.column.default.arg),
+                    sqlalchemy.sql.expression.literal(value),
                 )
             return op
 

tests/units/test_model.py

@@ -210,6 +210,110 @@ class AlembicThing(Model, table=True):
     assert len(list(versions.glob("*.py"))) == 6
 
 
+@pytest.mark.filterwarnings(
+    "ignore:This declarative base already contains a class with the same class name",
+)
+def test_automigration_add_column_with_callable_default(
+    tmp_working_dir: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    model_registry: type[ModelRegistry],
+):
+    """Test adding a column with a callable default to an existing table.
+
+    A callable default (e.g. ``default_factory=datetime.now``) must be evaluated
+    before it can be rendered as a SQL literal server_default for existing rows.
+
+    Args:
+        tmp_working_dir: directory where database and migrations are stored
+        monkeypatch: pytest fixture to overwrite attributes
+        model_registry: clean reflex ModelRegistry
+    """
+    import datetime
+
+    import sqlmodel
+
+    alembic_ini = tmp_working_dir / "alembic.ini"
+    versions = tmp_working_dir / "alembic" / "versions"
+    monkeypatch.setattr(reflex.constants, "ALEMBIC_CONFIG", str(alembic_ini))
+
+    config_mock = mock.Mock()
+    config_mock.db_url = f"sqlite:///{tmp_working_dir}/reflex.db"
+    monkeypatch.setattr(reflex.model, "get_config", mock.Mock(return_value=config_mock))
+
+    alembic_init()
+
+    class AlembicCallable(Model, table=True):  # pyright: ignore [reportRedeclaration]
+        t1: str
+
+    with get_engine().connect() as connection:
+        assert alembic_autogenerate(connection=connection, message="Initial Revision")
+    assert migrate()
+
+    with reflex.model.session() as session:
+        session.add(AlembicCallable(t1="existing"))
+        session.commit()
+
+    model_registry.get_metadata().clear()
+
+    # Add a non-nullable column with a callable default alongside a scalar
+    # default. Rendering the migration script previously raised CompileError
+    # because the callable was fed into sqlalchemy.literal(); both defaults must
+    # be carried as server defaults so the existing row can be migrated.
+    class AlembicCallable(Model, table=True):  # pyright: ignore [reportRedeclaration]
+        t1: str
+        created: datetime.datetime = sqlmodel.Field(
+            default_factory=datetime.datetime.now
+        )
+        count: int = 5
+
+    assert migrate(autogenerate=True)
+    assert len(list(versions.glob("*.py"))) == 2
+
+    now = datetime.datetime.now()
+    with reflex.model.session() as session:
+        session.add(AlembicCallable(t1="foo"))
+        session.commit()
+        result = session.exec(sqlmodel.select(AlembicCallable)).all()
+        assert len(result) == 2
+        assert result[0].t1 == "existing"
+        assert result[0].count == 5
+        assert result[0].created < now
+        assert result[1].t1 == "foo"
+        assert result[1].count == 5
+        assert result[1].created >= now
+
+    model_registry.get_metadata().clear()
+
+    # A nullable callable default is evaluated for existing rows and remains a
+    # Python-side default for new rows.
+    class AlembicCallable(Model, table=True):  # pyright: ignore [reportRedeclaration]
+        t1: str
+        created: datetime.datetime = sqlmodel.Field(
+            default_factory=datetime.datetime.now
+        )
+        count: int = 5
+        note: str | None = sqlmodel.Field(default_factory=lambda: "generated")
+
+    assert migrate(autogenerate=True)
+    assert len(list(versions.glob("*.py"))) == 3
+
+    with reflex.model.session() as session:
+        session.add(AlembicCallable(t1="bar"))
+        session.commit()
+        result = session.exec(sqlmodel.select(AlembicCallable)).all()
+        assert len(result) == 3
+        # Pre-existing rows receive the evaluated server default.
+        assert result[0].t1 == "existing"
+        assert result[0].note == "generated"
+        assert result[1].t1 == "foo"
+        assert result[1].note == "generated"
+        # Newly inserted row gets the callable default from sqlmodel.
+        assert result[2].t1 == "bar"
+        assert result[2].note == "generated"
+
+    model_registry.get_metadata().clear()
+
+
 class ReflexModel(Model):
     """A model for testing."""