What is the JSON format for a firestore composite index including an array?

Question

I want to filter a collection (let's call it documents) using array-contains on one column (say keywords) and sort by another column (say name).

I am able to create this composite index in the firebase console, but I can only guess at the format for adding it to firestore.indexes.json.

It's unfortunate we can't download the index file from the console.

Answer 1

Set the mode to ARRAY_CONTAINS:

{
      "collectionId": "documents",
      "fields": [
        {
          "fieldPath": "keywords",
          "mode": "ARRAY_CONTAINS"
        },
        {
          "fieldPath": "name",
          "mode": "ASCENDING"
        }
  ]
}

You can also list your current Cloud Firestore indexes in JSON from the Firebase CLi:

firebase firestore:indexes