What steps will reproduce the problem?

import cPickle
from google.appengine.ext import ndb

class TestModel(ndb.Model):
  foo = ndb.StringProperty()

model1 = TestModel(foo='bar', projection=('foo',))
model2 = cPickle.loads(cPickle.dumps(model1, -1))
print model1 == model2, model2._projection


What is the expected output?  True ('foo',)
What do you see instead?  False ()


What version of the product are you using? On what operating system?
1.7.3 on Mac OS X


Please provide any additional information below.

This is mildly problematic when putting projected entities in memcache, since 
when retrieved they no longer have the projection's safety constraints (no 
access to unprojected properties, no putting, etc.).

ndb.Model._from_pb appears to have some code to restore a projection by looking 
for meaning == entity_pb.Property.INDEX_VALUE, however _to_pb / 
ndb.Property._serialize don't set a meaning.  I assume this is because other 
users of _to_pb require that such a meaning not be written, so perhaps this 
calls for yet another flag on _to_pb?