doc.modify fails with complex Q objects

from mongoengine import EmbeddedDocument, Document, IntField, Q, EmbeddedDocumentField

class A(EmbeddedDocument):
    v = IntField()

class B(Document):
    a = EmbeddedDocumentField(A)

b = B.objects.create(a=A(v=1))
b.modify({'a__v': 1}, set__a__v=2)  # modify with dict works fine 

q = Q(a__v=2)
print list(B.objects(q))  # check that Q object is valid
b.modify(q, set__a__v=3)  # modify with Q object fails

Last line fails like this:

...
    b.modify(q, set__a__v=3)
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/document.py", line 250, in modify
    updated = self._qs(**query).modify(new=True, **update)
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 598, in modify
    query = queryset._query
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 1518, in _query
    self._mongo_query = self._query_obj.to_query(self._document)
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 89, in to_query
    query = query.accept(QueryCompilerVisitor(document))
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 155, in accept
    return visitor.visit_query(self)
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 78, in visit_query
    return transform.query(self.document, **query.query)
  File "/home/monitorius/rasp/.env/local/lib/python2.7/site-packages/mongoengine/queryset/transform.py", line 63, in query
    raise InvalidQueryError(e)
mongoengine.errors.InvalidQueryError: Cannot resolve field "a.v"

Reproduce the failure with the embedded-document example, then trace how Document.modify in mongoengine/document.py passes the Q object into QuerySet.modify in queryset/base.py. Read queryset/visitor.py and queryset/transform.py around field resolution, and compare this path with the working dictionary query. Done means the Q-based modify succeeds for the embedded field without regressing the existing query behavior.