Efficiently storing and getting likes in FireStore / Document DB

Question

I have a page with posts and likes for each post.

In FireStore a collection of posts and a collection of likes, and I update the total_likes and recent likes array when a user likes or unlikes a post with cloud functions.

However, I can't figure out how to show for each post if the currently logged in user liked it or not. What's an efficient way to do that for.

Any pointers?

Answer 1

I believe you might need to look at data aggregration. Even though this example is with Angular, I also use the same principle in a different application: https://angularfirebase.com/lessons/firestore-cloud-functions-data-aggregation/

Alternatively, you could store the post_id's that your user likes in their own 'like_array'. Knowing which posts the user currently sees, you can cross reference the shown post_id's with the (single object) 'like_array' from the user to determine if he/she has liked a particular post. In the long run, you could disambiguate like_arrays based on days or weeks, and only query the like_arrays of this and last day/week - based on what post you are showing. If you are working with categories of posts, similar applies: different like_arrays for different categories.

Hope this helps!

Answer 2

One solution would be to have another collection in your Firestore database where you create a document by user, in which you save (and update) an object containing all the posts this user has liked.

Like

- likers    (Collection)
   - UserUID (doc)
      -  postIds {
             post1_UID: true,
             post2_UID: true 
         }

The idea is to use the technique described in the doc, here: https://firebase.google.com/docs/firestore/solutions/arrays#solution_a_map_of_values

I don't know which language you use in the front end but in JavaScript you would do:

var postToTestId = ....;  <- You set this value as you need (e.g. as a function parameter)
firebase.auth().signInWithEmailAndPassword("...", ".....")
    .then(function (info) {
        var postId = 'azer';
        return db.collection('likers')
            .where('postIds.'+ postToTestId, '==', true)
            .get();
    })
    .then(function(querySnapshot) {
        if (querySnapshot.size > 0) {
            console.log("USER LIKES THIS POST!!!");
        }
    })
    .catch(function (error) {
        console.log(error);
    });

I don't think there is any solution without storing somewhere all the posts each user liked...