Google Datastore Order with ancestors

Question

Language: Python.

I'm using datastore python lib, everything works fine. When using datastore query to sort the query result, I can add query.order = ['name']; to sort the result. But when query a table with ancestors, like:

ancestor = client.key('user', name, namespace = NAMESPACE)
query = client.query(kind='project', ancestor=ancestor, namespace = NAMESPACE)

Then I set order: query.order = ['name'];, it doesn't work. I wanna sort on the kind project, whose ancestor is kind user. The error message is: "400 no matching index found. recommended index is:↵- kind: project↵ ancestor: yes↵ properties:↵ - name: name", which is a yaml sample. But I'm not using yaml here. I think there must be a way to sort the result though there's ancestor.

Answer 1

All datastore queries are index-based. From Indexes:

Every Google Cloud Datastore query computes its results using one or more indexes which contain entity keys in a sequence specified by the index's properties and, optionally, the entity's ancestors. The indexes are updated to reflect any changes the application makes to its entities, so that the correct results of all queries are available with no further computation needed.

The error you're getting comes from the Datastore side, indicating that the index required for your specific query is missing in the Datastore. It is unrelated to doing an ancestor query and/or to using yaml in your application code.

The yaml reference is simply coming from the way the indexes are being deployed to the Datastore (which is something you need to do for your query to work), see Deploying or deleting indexes.

So you need to create an index.yaml file containing at least the index specification indicated in the error message (and any other indexes you may need for other queries, if any), deploy it to the Datastore, wait for the index to get into the Serving state on the Indexes page (which may take a while), after which your query should work.