Raise an error if trying to perform a join

You can't join across reference fields, so raise an error
if someone tries to.

docs/changelog.rst

@@ -5,6 +5,7 @@ Changelog
 Changes in dev
 ==============
 
+- Errors raised if trying to perform a join in a query
 - Updates can now take __raw__ queries
 - Added custom 2D index declarations
 - Added replicaSet connection support

mongoengine/queryset.py

@@ -610,6 +610,9 @@ class QuerySet(object):
                     raise InvalidQueryError('Cannot resolve field "%s"'
                                                 % field_name)
             else:
+                from mongoengine.fields import ReferenceField, GenericReferenceField
+                if isinstance(field, (ReferenceField, GenericReferenceField)):
+                    raise InvalidQueryError('Cannot perform join in mongoDB: %s' % '__'.join(parts))
                 # Look up subfield on the previous field
                 new_field = field.lookup_member(field_name)
                 from base import ComplexBaseField

tests/document.py

@@ -10,6 +10,7 @@ from fixtures import Base, Mixin, PickleEmbedded, PickleTest
 
 from mongoengine import *
 from mongoengine.base import NotRegistered, InvalidDocumentError
+from mongoengine.queryset import InvalidQueryError
 from mongoengine.connection import get_db
 
 
@@ -640,7 +641,7 @@ class DocumentTest(unittest.TestCase):
             location = DictField()
             meta = {
                 'indexes': [
-                    '*location.point', 
+                    '*location.point',
                 ],
             }
         Place.drop_collection()
@@ -2256,6 +2257,22 @@ class DocumentTest(unittest.TestCase):
 
         BlogPost.drop_collection()
 
+    def test_cannot_perform_joins_references(self):
+
+        class BlogPost(Document):
+            author = ReferenceField(self.Person)
+            author2 = GenericReferenceField()
+
+        def test_reference():
+            list(BlogPost.objects(author__name="test"))
+
+        self.assertRaises(InvalidQueryError, test_reference)
+
+        def test_generic_reference():
+            list(BlogPost.objects(author2__name="test"))
+
+        self.assertRaises(InvalidQueryError, test_generic_reference)
+
     def test_duplicate_db_fields_raise_invalid_document_error(self):
         """Ensure a InvalidDocumentError is thrown if duplicate fields
         declare the same db_field"""