add custom Field processing for mixins used in the Model subclasses (#167)

mypy_django_plugin/transformers/fields.py

@@ -116,7 +116,9 @@ def transform_into_proper_return_type(ctx: FunctionContext, django_context: Djan
     assert isinstance(default_return_type, Instance)
 
     outer_model_info = helpers.get_typechecker_api(ctx).scope.active_class()
-    if not outer_model_info or not outer_model_info.has_base(fullnames.MODEL_CLASS_FULLNAME):
+    if (outer_model_info is None
+            or not outer_model_info.has_base(fullnames.MODEL_CLASS_FULLNAME)
+            and outer_model_info.fullname() not in helpers.get_all_model_mixins(helpers.get_typechecker_api(ctx))):
         # not inside models.Model class
         return ctx.default_return_type
     assert isinstance(outer_model_info, TypeInfo)

mypy_django_plugin/transformers/models.py

@@ -212,6 +212,18 @@ class AddMetaOptionsAttribute(ModelClassInitializer):
                                              ]))
 
 
+class RecordAllModelMixins(ModelClassInitializer):
+    def run(self) -> None:
+        basemodel_info = self.lookup_typeinfo_or_incomplete_defn_error(fullnames.MODEL_CLASS_FULLNAME)
+        basemodel_metadata = helpers.get_django_metadata(basemodel_info)
+        if 'model_mixins' not in basemodel_metadata:
+            basemodel_metadata['model_mixins'] = {}
+
+        for base_info in self.model_classdef.info.mro[1:]:
+            if base_info.fullname() != 'builtins.object':
+                basemodel_metadata['model_mixins'][base_info.fullname()] = 1
+
+
 def process_model_class(ctx: ClassDefContext,
                         django_context: DjangoContext) -> None:
     initializers = [
@@ -220,7 +232,8 @@ def process_model_class(ctx: ClassDefContext,
         AddRelatedModelsId,
         AddManagers,
         AddExtraFieldMethods,
-        AddMetaOptionsAttribute
+        AddMetaOptionsAttribute,
+        RecordAllModelMixins,
     ]
     for initializer_cls in initializers:
         try: