Reflect Inheritance in Field's 'owner_document'

The 'owner_document' property of a Field now reflects the parent field
which first contained the Field when a Document in inherited.

Fixes #954
Closes #955

mongoengine/base/fields.py

@@ -83,6 +83,7 @@ class BaseField(object):
         self.help_text = help_text
         self.null = null
         self.sparse = sparse
+        self._owner_document = None
 
         # Adjust the appropriate creation counter, and save our local copy.
         if self.db_field == '_id':
@@ -189,6 +190,17 @@ class BaseField(object):
 
         self.validate(value, **kwargs)
 
+    @property
+    def owner_document(self):
+        return self._owner_document
+
+    def _set_owner_document(self, owner_document):
+        self._owner_document = owner_document
+
+    @owner_document.setter
+    def owner_document(self, owner_document):
+        self._set_owner_document(owner_document)
+
 
 class ComplexBaseField(BaseField):
 
@@ -398,11 +410,6 @@ class ComplexBaseField(BaseField):
             self.field.owner_document = owner_document
         self._owner_document = owner_document
 
-    def _get_owner_document(self, owner_document):
-        self._owner_document = owner_document
-
-    owner_document = property(_get_owner_document, _set_owner_document)
-
 
 class ObjectIdField(BaseField):
 

mongoengine/base/metaclasses.py

@@ -176,7 +176,8 @@ class DocumentMetaclass(type):
         # Handle delete rules
         for field in new_class._fields.itervalues():
             f = field
-            f.owner_document = new_class
+            if f.owner_document is None:
+                f.owner_document = new_class
             delete_rule = getattr(f, 'reverse_delete_rule', DO_NOTHING)
             if isinstance(f, CachedReferenceField):