title: Key Path Element Must be complete
body: <p>I'm working through some documentation for the google cloud datastore API</p>
  
  <pre><code>from google.cloud import datastore
  datastore_client = datastore.Client(project="PROJECTNAME")
  query = datastore_client.query(kind='Video')
  
  r = query.fetch()
  for v in r:
    key = datastore_client.key('VideosToCollections')
    entity = datastore.Entity(key=key)
  
    entity['collection_key'] = key
    datastore_client.put(entity)
    quit()
  </code></pre>
  
  <p>However I  receive this error when updating an entry</p>
  
  <pre><code>Traceback (most recent call last):
  File "fillvideocollections.py", line 20, in &lt;module&gt; datastore_client.put(entity)
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/client.py", line 421, in put
  self.put_multi(entities=[entity])
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/client.py", line 448, in put_multi
  current.commit()
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/batch.py", line 274, in commit
  self._commit()
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore/batch.py", line 250, in _commit
  self.project, mode, self._mutations, transaction=self._id
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/datastore_v1/gapic/datastore_client.py", line 501, in commit
  request, retry=retry, timeout=timeout, metadata=metadata
  File "/usr/local/lib/python2.7/dist-packages/google/api_core/gapic_v1/method.py", line 143, in __call__
  return wrapped_func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/google/api_core/retry.py", line 270, in retry_wrapped_func
  on_error=on_error,
  File "/usr/local/lib/python2.7/dist-packages/google/api_core/retry.py", line 179, in retry_target
  return target()
  File "/usr/local/lib/python2.7/dist-packages/google/api_core/timeout.py", line 214, in func_with_timeout
  return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/google/api_core/grpc_helpers.py", line 59, in error_remapped_callable
  six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "/home/jeffbrubaker/.local/lib/python2.7/site-packages/six.py", line 737, in raise_from
  raise value
  google.api_core.exceptions.InvalidArgument: 400 Key path element must not be incomplete: [VideosToCollections: ]
  </code></pre>
  
  <p>I want the code above to create one VideosToCollections object as a stepping stone, but will expand to creating one per Video. Any thoughts on what is causing the above error would be appreciated.</p>