How to filter a MongoDB/Mongoose query based upon referenced subdocuments values

Question

So I am currently working on cloning some app concepts (for learning purposes) and I have built a basic social media app that has users, posts, & allows users to block users if they don't want to share data. But my question is, can I use referenced documents values/data (a populated user's array of blocked users) to filter out the parent document (the post)? Hope this makes sense.

So far, here are the schemas (trimmed down of course):

// User Schema
const mongoose = require('mongoose');
const Schema = mongoose.Schema;    
const UserSchema = new Schema({
    email: {
        type: String,
        required: [true, "Please provide an email"],
        unique: true
    },
    password: {
        type: String,
        minlength: 8
    },
    blockedUsers: {
        type: [{
            type: Schema.Types.ObjectId,
            ref: 'User'
        }]
    }
});

// Post Schema
const PostSchema = new Schema({
    author: {
        type: Schema.Types.ObjectId,
        ref: 'User'
    },
    dateCreated: {
        type: Date,
        default: Date.now()
    },
    body: {
        type: String,
        required: true,
        trim: true,
        maxlength: 1200
    }
});

const User = mongoose.model('User', UserSchema);
const Post = mongoose.model('Post', PostSchema);

Then my query I have so far which only filters out the subdocument and in my tests is returning a null author to my post which is not exactly the functionality I was hoping for. To clarify, it still returns the right post that was queried, but it does not populate the 'author' field and instead sets the author field to null .

router.get('/p/:postID', catchAsyncErr(async (req, res, next) => {
    // Find post and make sure the user requesting the post is not blocked by the author
    const post = await Post.findById(req.params.postID).populate({
        path: 'author',
        select: 'blockedUsers',
        match: {
            blockedUsers: {
                $ne: req.user.id // The user ID stored in the JSON Web token
            }
        }
    });

    res.status(200).json({
        status: 'Success',
        statusCode: 200,
        message: `Post "${req.params.postID}" found`,
        data: post
    })
}));

I tried using aggregation and honestly am still trying to fully wrap my head around the $lookup concept. But I have a strange feeling that that should be where I am testing more...

Basically, I am really trying to hide the post and return a 404 if the author (user) has the requesting user on the 'blockedUsers' list. I understand I can do this using regular if/else statements but I was really hoping to be able to do this inside a query. Further more, I want to be able to implement this same concept on a users home feed if you will. If they are blocked by a user, the user who blocked them would not have their post show up in their feed.

Thank you for any of your insights on this!

Answer 1

You can use aggregation pipeline to do that. You need $lookup for join (populate) author and $match for filter your user.

$match is very similar to your first parameter in find() .

$lookup is similar to join in SQL if you familiar with.

For setting up $lookup you need four parameters.

from : Which is your destination collection (Here is users ) localField : Which your foreign key field name (Here author ) foreignField : Which is destination collection field (Here _id ) as : Name after join (Here I use author )

Here is the snippet:

const post = Post.aggregate([{
  $match: {
    _id: req.params.postID
  }
}, 
{
  $lookup: {
    from: 'users',
    localField: 'author',
    foreignField: '_id',
    as: 'author'
  }
}])

Then your post.author.blockedUsers should work fine.

If you want to retrieve specific blockedUsers you can use aggregation like this:

const post = Post.aggregate([{
  $match: {
    _id: req.params.postID
  }
}, 
{
  $lookup: {
    from: 'users',
    'let': {authorId: "$author"},
    pipeline: [{
      $match: {
        $expr: {
          $and: [{
            $eq: ['$_id', '$$authorId']
          }, {
            $ne: ['$blockedUsers', req.user.id]
          }]
        }
      }
    }
    as: 'author'
  }
}])