Chained queries break on same fields ($in followed by $eq).

[anemitz]

        class A(Document):                                                                                                              
            pass                                                                                                                        

        class B(Document):                                                                                                              
            a = ReferenceField(A)                                                                                                       
            x = BooleanField()                                                                                                          

        a1 = A().save()                                                                                                                 
        a2 = A().save()                                                                                                                                                                                                                       

        # Works                                                  
        qs = B.objects.filter(a__in=[a1,a2], a=a1)                                                                                      

        # Doesn't work
        qs = B.objects.filter(a__in=[a1,a2], a=a1)                                                                                      
        qs = qs.filter(a=a1)   

  File "/Users/anemitz/Dropbox/projects/mongoengine/mongoengine/queryset.py", line 387, in _query
    self._mongo_query = self._query_obj.to_query(self._document)
  File "/Users/anemitz/Dropbox/projects/mongoengine/mongoengine/queryset.py", line 210, in to_query
    query = self.accept(SimplificationVisitor())
  File "/Users/anemitz/Dropbox/projects/mongoengine/mongoengine/queryset.py", line 261, in accept
    return visitor.visit_combination(self)
  File "/Users/anemitz/Dropbox/projects/mongoengine/mongoengine/queryset.py", line 80, in visit_combination
    return Q(**self._query_conjunction(queries))
  File "/Users/anemitz/Dropbox/projects/mongoengine/mongoengine/queryset.py", line 95, in _query_conjunction
    raise InvalidQueryError(msg + ', '.join(intersection))
InvalidQueryError: Duplicate query conditions: a

[rozza]

You define a and match against it multiple times - which is why the second example complains. Arguably, it should merge and make them all $in statements.

[anemitz]

I'm patching it to do this: {$and: [{x: {$in:[1,2]}}, {x:1}]}

[rozza]

yeah that sounds better

[rozza]

Ok this passes for me..