Multi tool use

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

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, and each blog post has categories.

Post {
title: ..
...
categories: {
cats: true
puppies: true
}
}


In order to query posts in a particular category in a paginated way, we would do something like this:

let query = db.collection(`/posts`)
.where(`categories.${categoryId}`, '==', true)
.orderBy('createdAt')
.startAfter(lastDate)
.limit(5);


But it seems that this would require a composite index (categories.<categoryId> and createdAt) for every single category. Is there any way around this?

categories.<categoryId>

createdAt

In my case, it isn't feasible to create composite indices for every category since categories are user-generated, and could easily exceed 200 (the limit for composite indices in Firestore).

3 Answers
3

This is doable by setting the value of each category to what you want to sort on. Firestore has a guide that covers this.

Post {
title: ..
...
categories: {
cats: createdAt
puppies: createdAt
}
}

let query = db.collection(`/posts`)
.where(`categories.${categoryId}`, '>', 0)
.orderBy(`categories.${categoryId}`)
.startAfter(lastDate)
.limit(5);


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).

orderBy

startAfter

orderBy

Try restructuring your data store. Firebase documentation is very helpful here.

Cloud Firestore does not support the following types of queries:

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