Added InvalidDocumentError

Ensures defined documents are valid and users don't override core methods
by accident.

fixes #275

mongoengine/base.py

@@ -16,6 +16,9 @@ class NotRegistered(Exception):
     pass
 
 
+class InvalidDocumentError(Exception):
+    pass
+
 class ValidationError(Exception):
     pass
 
@@ -388,6 +391,8 @@ class DocumentMetaclass(type):
         attrs['_db_field_map'] = dict([(k, v.db_field) for k, v in doc_fields.items() if k!=v.db_field])
         attrs['_reverse_db_field_map'] = dict([(v, k) for k, v in attrs['_db_field_map'].items()])
 
+        from mongoengine import Document
+
         new_class = super_new(cls, name, bases, attrs)
         for field in new_class._fields.values():
             field.owner_document = new_class
@@ -396,6 +401,9 @@ class DocumentMetaclass(type):
                 field.document_type.register_delete_rule(new_class, field.name,
                         delete_rule)
 
+            if field.name and hasattr(Document, field.name):
+                raise InvalidDocumentError("%s is a document method and not a valid field name" % field.name)
+
         module = attrs.get('__module__')
 
         base_excs = tuple(base.DoesNotExist for base in bases