make EmbeddedDocument not hashable by default (#1528)

mongoengine/base/document.py

@@ -272,13 +272,6 @@ class BaseDocument(object):
     def __ne__(self, other):
         return not self.__eq__(other)
 
-    def __hash__(self):
-        if getattr(self, 'pk', None) is None:
-            # For new object
-            return super(BaseDocument, self).__hash__()
-        else:
-            return hash(self.pk)
-
     def clean(self):
         """
         Hook for doing document level data cleaning before validation is run.

mongoengine/document.py

@@ -60,6 +60,12 @@ class EmbeddedDocument(BaseDocument):
     my_metaclass = DocumentMetaclass
     __metaclass__ = DocumentMetaclass
 
+    # A generic embedded document doesn't have any immutable properties
+    # that describe it uniquely, hence it shouldn't be hashable. You can
+    # define your own __hash__ method on a subclass if you need your
+    # embedded documents to be hashable.
+    __hash__ = None
+
     def __init__(self, *args, **kwargs):
         super(EmbeddedDocument, self).__init__(*args, **kwargs)
         self._instance = None
@@ -160,6 +166,15 @@ class Document(BaseDocument):
         """Set the primary key."""
         return setattr(self, self._meta['id_field'], value)
 
+    def __hash__(self):
+        """Return the hash based on the PK of this document. If it's new
+        and doesn't have a PK yet, return the default object hash instead.
+        """
+        if self.pk is None:
+            return super(BaseDocument, self).__hash__()
+        else:
+            return hash(self.pk)
+
     @classmethod
     def _get_db(cls):
         """Some Model using other db_alias"""