CODEWITHSUNDEEP
firebasereact-nativegoogle-cloud-firestore
Imran Ali
Imran Ali

Reputation: 91

Firestore, fetch only those documents from a collection which are not present in client cache

I am implementing a one-to-one chat app using firestore in which there is a collection named chat such that each document of a collection is a different thread.

When the user opens the app, the screen should display all threads/conversations of that user including those which have new messages (just like in whatsapp). Obviously one method is to fetch all documents from the chat collection which are associated with this user.

However it seems a very costly operation, as the user might have only few updated threads (threads with new messages), but I have to fetch all the threads.

Is there an optimized and less costly method of doing the same where only those threads are fetched which have new messages or more precisely threads which are not present in the user's device cache (either newly created or modified threads).

Each document in the chat collection have these fields:

senderID: (id of the user who have initiated the thread/conversation)
receiverID: (id of the other user in the conversation)
messages: [],
lastMsgTime: (timestamp of last message in this thread)

Currently to load all threads of a certain user, I am applying the following query:

const userID = firebase.auth().currentUser.uid
firebase.firestore().collection('chat').where('senderId', '==', userID)
firebase.firestore().collection('chat').where('receiverId', '==', userID)

and finally I am merging the docs returned by these two queries in an array to render in a flatlist.

Upvotes: 0

Views: 146

Answers (1)

Frank van Puffelen
Frank van Puffelen

Reputation: 598668

In order to know whether a specific thread/document has been updated, the server will have to read that document, which is the charged operation that you're trying to avoid.

The only common way around this is to have the client track when it was last online, and then do a query for documents that were modified since that time. But if you want to show both existing and new documents, this would have to be a separate query, which means that it'd end up in a separate area of the cache. So in that case you'll have to set up your own offline storage on top of Firestore's, which is more work than I'm typically willing to do.

Upvotes: 1

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