feat: named database support

[Mariatta]

The branch multi-db was outdated. This PR makes it up to date with main and contains the commit from #398.

We will create the multi db test parametrization PR (#436) branch against this branch instead of main.

Once the feature is out of preview, we should:

• merge feat: Multi db test parametrization #436 into this branch
• merge this PR into main.

[Mariatta]

The http test is failing here, but it gets fixed by #436. So once we merge #436 into this branch, everything will be good.

[vishwarajanand]

I've updated these PRs and got the tests green. Seeking a go ahead for merging this.

[kolea2]

LGTM after nits and pending approval from @googleapis/yoshi-python

[parthea]

Wait for #436

[smittysmee]

@Mariatta - how sure are we on regression here? (Support case now open)

Unhandled exception: 'Key' object has no attribute '_database'
...
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/_options.py", line 102, in wrapper
    return wrapped(*pass_args, **kwargs)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/utils.py", line 118, in wrapper
    return wrapped(*args, **new_kwargs)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/utils.py", line 150, in positional_wrapper
    return wrapped(*args, **kwds)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/model.py", line 5389, in _put
    return self._put_async(_options=kwargs["_options"]).result()
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 210, in result
    self.check_success()
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 157, in check_success
    raise self._exception
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 319, in _advance_tasklet
    yielded = self.generator.throw(type(error), error, traceback)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/model.py", line 5450, in put
    ds_key = yield _datastore_api.put(ds_entity, kwargs["_options"])
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 323, in _advance_tasklet
    yielded = self.generator.send(send_value)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/ndb/_datastore_api.py", line 371, in put
    entity_pb = helpers.entity_to_protobuf(entity)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/helpers.py", line 216, in entity_to_protobuf
    _set_protobuf_value(value_pb, value)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/helpers.py", line 476, in _set_protobuf_value
    entity_pb = entity_to_protobuf(val)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/helpers.py", line 216, in entity_to_protobuf
    _set_protobuf_value(value_pb, value)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/helpers.py", line 470, in _set_protobuf_value
    attr, val = _pb_attr_value(val)
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/helpers.py", line 360, in _pb_attr_value
    name, value = "key", val.to_protobuf()
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/key.py", line 310, in to_protobuf
    if self.database:
  File "/layers/google.python.pip/pip/lib/python3.7/site-packages/google/cloud/datastore/key.py", line 421, in database
    return self._database
AttributeError: 'Key' object has no attribute '_database'

[parthea]

Hi @smittysmee,

Please could you file a bug with sample code? Please include the versions of google-cloud-ndb and google-cloud-datastore in the bug to help us re-create the issue.

[smittysmee]

@parthea - it was a lack of backwards compatibility in a rollover and cache.

• Previous version - get entity - update cache with entity A.
• Released new version - cache hit of entity A - perform creation of entity B with reference to entity A, error.

Entity A did not update with latest key structure containing added attribute _database

[vishwarajanand]

@smittysmee by cache hit, it suggests to me that the issue is local to your environment? Pls confirm if that was not the case.
Any sample to reproduce the issue like Parth suggested will be superhelpful.