look for models.Model in full mro for to= parameter

mypy_django_plugin/plugins/related_fields.py

@@ -48,10 +48,7 @@ def get_valid_to_value_or_none(ctx: FunctionContext) -> Optional[Instance]:
     referred_to_type = arg_type.ret_type
     if not isinstance(referred_to_type, Instance):
         return None
-    for base in referred_to_type.type.bases:
-        if base.type.fullname() == helpers.MODEL_CLASS_FULLNAME:
-            break
-    else:
+    if not referred_to_type.type.has_base(helpers.MODEL_CLASS_FULLNAME):
         ctx.api.msg.fail(f'to= parameter value must be '
                          f'a subclass of {helpers.MODEL_CLASS_FULLNAME}',
                          context=ctx.context)

test-data/typecheck/related_fields.test

@@ -228,3 +228,14 @@ class User(models.Model):
     friends = models.ManyToManyField('self', on_delete=models.CASCADE)
 reveal_type(User().friends)  # E: Revealed type is 'django.db.models.manager.RelatedManager[main.User*]'
 [out]
+
+[CASE recursively_checking_for_base_model_in_to_parameter]
+from django.db import models
+
+class BaseModel(models.Model):
+    pass
+class ParkingSpot(BaseModel):
+    pass
+class Booking(BaseModel):
+    parking_spot = models.ForeignKey(to=ParkingSpot, null=True, on_delete=models.SET_NULL)
+[out]