In Firestore, how can you do a compound query involving a key in a map without creating an index for every key?

Question

In Firestore, how can you do a compound query involving a key in a map without creating an index for every key?

For example, consider a collection which holds blog posts

Answer 1

As far as I know Firestore should auto-generate those indexes. From the documentation page on arrays, lists, and sets:

Consider this alternative data structure, where each category is the key in a map and all values are true:

// Sample document in the 'posts' collection
{
    title: "My great post",
    categories: {
        "technology": true,
        "opinion": true,
        "cats": true
    }
}

Now it's easy to query for all blog posts within a single category:

// Find all documents in the 'posts' collection that are
// in the 'cats' category.
db.collection('posts')
    .where('categories.cats', '==', true)
    .get()
    .then(() => {
        // ...
    });
)

This technique relies on the fact that Cloud Firestore creates built-in indexes for all document fields, even fields in a nested map.

While the lefthand-side of your where condition may be variable, that doesn't change the fact that these indexes should auto-generated (as far as I can see).