Made type inheritance a validation check for abstract references

mongoengine/fields.py

@@ -996,6 +996,14 @@ class ReferenceField(BaseField):
             self.error('You can only reference documents once they have been '
                        'saved to the database')
 
+        if self.document_type._meta.get('abstract') and \
+                not isinstance(value, self.document_type):
+            self.error('%s is not an instance of abstract reference'
+                    ' type %s' % (value._class_name,
+                        self.document_type._class_name)
+                    )
+
+
     def lookup_member(self, member_name):
         return self.document_type._fields.get(member_name)