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I'm working on a Google App Engine application in python. I tried to switch a query I was running for one of my routes to query on only one property instead of 2, which caused an indexing error to appear whenever we tried running that query.
It was something along the lines of "No index matching the specified parameters could be found", but I don't have any screen shots at the moment. In order to try and rectify the situation, we ran appcfg.py vacuum_indices and deleted all indices related to the original search. We then uploaded a new index.yaml specifying the new index. Though we were able to see that the new indexes had indeed been created on the admin panel, and the old ones were gone, we were still getting the same error.
We're really unsure why this is happening, and are having trouble finding documentation online for these issues. Our next thought was that some previous state in memcache was causing the query to attempt to use it's old index. So we flushed memcache, and now we're getting this error:
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 1535, in __call__
rv = self.handle_exception(request, response, e)
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 1529, in __call__
rv = self.router.dispatch(request, response)
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 1278, in default_dispatcher
return route.handler_adapter(request, response)
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 1102, in __call__
return handler.dispatch()
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 572, in dispatch
return self.handle_exception(e, self.app.debug)
File "/base/data/home/runtimes/python27/python27_lib/versions/third_party/webapp2-2.5.2/webapp2.py", line 570, in dispatch
return method(*args, **kwargs)
File "/base/data/home/apps/s~dev-erpetcloud2/dev1.392600188150722624/routes/users.py", line 172, in post
res_dict = cp_user.to_dict()
File "/base/data/home/apps/s~dev-erpetcloud2/dev1.392600188150722624/routes/models/../models/cp_models.py", line 248, in to_dict
animal_dict = animal.to_dict()
File "/base/data/home/apps/s~dev-erpetcloud2/dev1.392600188150722624/routes/models/../models/cp_models.py", line 574, in to_dict
protocol, params = self.get_protocol_and_params()
File "/base/data/home/apps/s~dev-erpetcloud2/dev1.392600188150722624/routes/models/../models/cp_models.py", line 395, in get_protocol_and_params
record = self.protocol_state_key.get()
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/key.py", line 572, in get
return self.get_async(**ctx_options).get_result()
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 342, in get_result
self.check_success()
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 389, in _help_tasklet_along
value = gen.send(val)
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/context.py", line 765, in get
pbs = entity._to_pb(set_key=False).SerializePartialToString()
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 3158, in _to_pb
self._check_initialized()
File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 3014, in _check_initialized
'Entity has uninitialized properties: %s' % ', '.join(baddies))
BadValueError: Entity has uninitialized properties: title
Looking through the datastore, the entity that this trace references definitely does have a 'title' property.
I've looked around a lot for errors that can arise from deleting indices and flushing memcache, and nothing useful has come up.
If someone could perhaps give me a bit of insight into what could be happening here and how these systems work (my mental model might be off), or point me in the right direction, that would be fantastic. Thanks!!

This error signifies that the property 'title' has been specified as a required property but you are trying to write an entity to datastore without initializing this property. This error occurs only at the time of put(). By any chance, did you make any changes in the entity definition, or a part of code which writes these entities to datastore ?
Edit: The error can also happen while trying to read an entity which has no value specified for a 'required' property.