Deleting collections in Firebase's Firestore

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·3 min read

Firebase's Firestore organizes data in collections and documents; a Firestore collection is simply a group of documents, like a simple folder.

Collections and documents can be represented using a filesystem-like path: for example, imagine we have a collection named users and a user with ID 1, its path will be /users/1. We can use this path to query or select a reference from Firestore.

One quirky thing to know about Firestore is that we cannot delete collections using a simple API. You'd expect Firestore to provide methods such as collection.delete() to drop the entire collection or subcollection; yet, it does not.

In order to delete a collection, you're required to delete every document in the collection. You can use batch deletions, but you are still limited to the number of documents in each batch. In the case of huge collections, you will have to delete several batches of documents.

Furthermore, if you delete a document containing sub-collections, these will not be deleted automatically. Weird, huh?

Deleting Firestore sub-collections using Firebase tools

The package firebase-tools has a nifty utility that allows deleting entire paths from your Firestore database.

// recursive-delete.ts

const tools = require('firebase-tools');

export async function recursiveDelete(
    path: string
): Promise<boolean> {
  return tools.firestore.delete(path, {
    project: process.env.GCLOUD_PROJECT,
    recursive: true,
    yes: true,
  });
}

const usersPath = `/users/1`;

// imagine each user has a sub-collection `posts`
// for example `/users/1/posts/2`
// all the documents under `/users/1/posts/` will also be deleted

recursiveDelete(usersPath)
    .then((done: boolean) => {
        // done
    })

Now, that's great and convenient. BUT. USE. WITH. CAUTION..

Again, this is a very disruptive operation. It will delete the document and all the subcollections of this document. And because it's running in an admin environment, no Firestore rule can help you.

Be very, very careful if you're planning on running this command: add the proper guards, check the user permissions programmatically and handle errors gracefully.

A word on pricing

Does deleting a sub-collection result in only 1 Firestore deletion?

No. Unfortunately, if your collection contains 5000 documents, and you delete the entire collection no matter how many operations you execute, Firebase will charge for 5000 deletions.

How to avoid this? Well, it depends on your data structure - and if you can tweak it according to your needs.

For example: does the collection need to be a collection, or could it be part of a document as an array of objects? If the potential size of the array is limited, this could be your answer.

With that said, there are other things to take into account:

  • Documents are limited to a size of 1MB; getting even near this number would almost certainly be a mistake
  • Firestore also charges for network transfer: if your array has a large number of objects and gets queried very often, you could end up paying even greater charges than having a subcollection, which, instead, can be paginated

Ultimately, it depends.

Final Words

Firestore does not have an API to delete entire collections and sub-collections: you're supposed to batch-delete all the documents within a collection and repeat the operation of a document contains sub-collections.

It makes sense, but it's far from being a great DX (Developer Experience).

We can use the recursiveDelete delete utility above to bypass this limitation using a firebase-tools built-in command that makes it as easy as pie.

With that said, this is a potentially very disruptive and expensive command; it needs to be executed with the proper guards.

If you're using this to save money on the number of deletions, be warned: Firestore will always charge you based on the number of documents deleted, not the number of operations.

If the operation above deletes ten thousand documents, Firebase will bill you for ten thousand document deletions.

If this sounds exaggerated, you will have to restructure your database.

If you have any questions, do not hesitate to send me an email!


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