demo of how to make a query_pb2 that only fetches keys (use a projection)

keys_only_query_with_query_pb2.py

from apache_beam.io.gcp.datastore.v1 import helper
ds = helper.get_datastore('pgdragonsong-dev')
from google.cloud.proto.datastore.v1 import datastore_pb2, entity_pb2, query_pb2
req = datastore_pb2.RunQueryRequest()
partition = entity_pb2.PartitionId()
partition.project_id = 'pgdragonsong-dev'
req.partition_id.CopyFrom(partition)
q = query_pb2.Query()
q.kind.add().name = 'TmpForDeletion'

# how to change entity_result_type from full to keys-only?
from googledatastore import helper as datastore_helper
datastore_helper.add_projection(q, '__key__')

req.query.CopyFrom(q)
ds.run_query(req)  # returns datastore_pb2.RunQueryResponse