How to create a nested matrix with different number of steps?

This is our current matrix for running tests:

strategy:
  fail-fast: false
  matrix:
    app:
      - contra-api
      - contra-database
      - contra-temporal-job-worker
      - contra-web-app
    shardIndex: [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
    shardTotal: [15]

It runs tests using 15 jobs for each app.

However, I would like to adjust it so that contra-api runs with 15 shards, but other services run with 5 shards.

I would imagine that something like:

- app: contra-api
  shardIndex:
    - 1
    - ...
  shardTotal: 15
- app: contra-database
  ...

would be possible, though I was not able to figure out the syntax

Microsoft Github Actions matrices are per one single object of array properties with the addition of the include: property, which is an array of objects with null/boolean/number/string properties.

That include: property will add to n-dimensional matrix defined by the array properties and each object in the include: array should have the same properties as defined by the matrix array properties. Therefore you can just write all wanted additional combinations out under the include: property.

All services/apps run with 5 shards, and the contra-api with 15 shards:

  matrix:
    app:
      - contra-api
      - contra-database
      - contra-temporal-job-worker
      - contra-web-app
    shardIndex: [1,2,3,4,5]
    shardTotal: [5]
    include:
      - app: contra-api
        shardIndex: 6
        shardTotal: 15
      - app: contra-api
        shardIndex: 7
        shardTotal: 15
      - app: contra-api
        shardIndex: 8
        shardTotal: 15
      # ...
      - app: contra-api
        shardIndex: 13
        shardTotal: 15
      - app: contra-api
        shardIndex: 14
        shardTotal: 15
      - app: contra-api
        shardIndex: 15
        shardTotal: 15