Datastore Index Not working for property that is JSON string

Question

The following query in Datastore return the expected number of results:

SELECT timestamp from leadgenie_campaign_model_dev where campaign = "3667f39d-a3ff-4acb-b1ca-6f730bbc7989"

This query is backed by an index, which allows for the projection.

But this one doesn't return any results, despite the fact that an index also exists. attrs is a JSON string

SELECT timestamp, attrs from leadgenie_campaign_model_dev where campaign = "3667f39d-a3ff-4acb-b1ca-6f730bbc7989"

Here's the spec for the indexes:

indexes:

- kind: leadgenie_campaign_model_dev
  properties:
  - name: campaign
  - name: attrs
  - name: timestamp

- kind: leadgenie_campaign_model_dev
  properties:
  - name: campaign
  - name: timestamp
  - name: attrs

- kind: leadgenie_campaign_model_dev
  properties:
  - name: campaign
  - name: timestamp

Answer 1

The solution was to include the subproperty in the query, after creating an index for those properties specifically.

SELECT timestamp, attrs.score, attrs.sz from leadgenie_campaign_model_dev where campaign = "3667f39d-a3ff-4acb-b1ca-6f730bbc7989"

google.api_core.exceptions.FailedPrecondition: 400 no matching index found. recommended index is:
- kind: leadgenie_campaign_model_dev
  properties:
  - name: campaign
  - name: attrs.score
  - name: attrs.sz
  - name: timestamp