How many ids can be stored in a field array in firestore collection

Question

I'm looking for a document design to store user ids that other users liked. My first approach (maybe not the best) is an array to store all users ids that liked someone. Something like:

/users/1
 - likedByUserIds: [1,2,3,4,5,...,1000,1000001,...]

I have to query by not liked users yet and show them first. So how many ids can be stored in a field array in firestore collection? It can be many user ids from previous/past likes.

Answer 1

The number of elements of the array is not fixed. The limit you will run into is the max total size of the document (1MB), which is documented here . The total size of the document is not going to be based on a single field. It will be based on everything you put in it. You can estimate the size of a document using information here .

For lists of data that are not bounded within the limits of a single document, it's better to store the items as individual docments in a collection. There is no bound to the number of documents in a collection.

Answer 2

Just what @Doug stated, "there's a maximum total size for a document: 1MB". There's also a Firestore single-field index exemption limit which is 200. A single-field index stores a sorted mapping of all the documents in a collection that contain a specific field. Each entry in a single-field index records a document's value for a specific field and the location of the document in the database.

Additionally, there's a single-field index exemptions that you can exempt a field from your automatic indexing settings by creating a single-field index exemption. An indexing exemption overrides the database-wide automatic index settings. Since we need to hold that information in memory of every process that could handle a write to your database, and compare the incoming write to this list of exemptions. This creates a need for both memory management and latency impact.

For further reference, you can check this related post about Firestore single-field index exemption limit.

Answer 3

Here a solution I found to avoid full search in firebase to perform get users that haven't liked yet .

• My users collection in firestore
• Add the user to redis add('users',userId)
• Add liked user to redis zAdd('liked_users_${uid}',{value: userId})
• Generate not liked yet zDiffStore('not_liked_yet',['users','liked_users_${uid}']
• Get not liked yet zRange('not_liked_yet', 0, -1)

It just worked !