Column name that contains the type identifier for polymorphic deserialization.
This should be set to the name of the database column that stores the type identifier string (e.g., "base_type", "type", "class_type").
The column must be included in the SELECT query results for polymorphic deserialization to work. The value in this column will be looked up in `typemap` to determine which GObject type to instantiate.
See `typemap` documentation for a complete example.