Writes all submissions to datastore.

def _write_to_datastore(self):
    """Writes all submissions to datastore."""
    # Populate datastore
    roots_and_submissions = zip([ATTACKS_ENTITY_KEY,
                                 TARGET_ATTACKS_ENTITY_KEY,
                                 DEFENSES_ENTITY_KEY],
                                [self._attacks,
                                 self._targeted_attacks,
                                 self._defenses])
    client = self._datastore_client
    with client.no_transact_batch() as batch:
      for root_key, submissions in roots_and_submissions:
        batch.put(client.entity(client.key(*root_key)))
        for k, v in iteritems(submissions):
          entity = client.entity(client.key(
              *(root_key + [KIND_SUBMISSION, k])))
          entity['submission_path'] = v.path
          entity.update(participant_from_submission_path(v.path))
          batch.put(entity)