Datastore: When runQuery fails with index required, the exact index needed should be part of the error message.

This is the current error message when a required index is not found:

ApiError: Precondition Failed - no matching index found.

Many lives would be immeasurably improved if the error looked more like this:

ApiError: Precondition Failed - no matching index found.
    Solution: You must add the following index to your index.yaml file:
        - kind: Cat
          ancestor: no
          properties:
          - name: name
          - name: age
            direction: desc
    For more info, see: https://cloud.google.com/sdk/gcloud/reference/preview/datastore/create-indexes

For instance, this is the query I am currently trying to run:

{
    "namespace":null,
    "kinds":["Task"],
    "filters":[
        {
            "name":"__key__",
            "op":"HAS_ANCESTOR",
            "val":{
                "name":"9CDFB82D-FC98-2B7F-22C5-A391A0623474",
                "kind":"Project",
                "path":["Project","9CDFB82D-FC98-2B7F-22C5-A391A0623474"]
            }
        },
        {"name":"parentId =","op":"=","val":"9"}
    ],
    "orders":[{"name":"order","sign":"+"}],
    "groupByVal":[],
    "selectVal":[],
    "autoPaginateVal":true,
    "startVal":null,
    "endVal":null,
    "limitVal":-1,
    "offsetVal":-1
}

I get the Error that no matching index is found.
My application has the following index specified:

Task
Includes ancestors parentId ▲ + order ▲

This leaves me unsure of what the problem is, or what index I need to add.

@pcostell

@pcostell - is it possible/reliable to generate such an error message given the query object and 412 response?

Yup this has been a rough experience. We are working on adding this to the server error message, so as long as you pass along the error message this should start working soon.