Cascade delete from array using Mongoose middleware remove hook

Question

I am building a Node.js express RESTfull API using Mongodb and mongoose.

This is my schema:

var UserSchema = new mongo.Schema({
  username: { type: String },
  password: { type: String, min: 8 },
  display_name: { type: String, min: 1 },
  friends: { type: [String] }
});

UserSchema.post('remove', function(next){
  console.log({ friends: this._id }); // to test if this gets reached (it does)
  UserSchema.remove({ friends: this._id });
});

And this is the function that removes a User:

.delete(function(req, res) {
  User.findById(req.params.user_id, function(err, user) {
    if (err) {
      res.status(500);
      res.send(err);
    } else {
      if (user != null) {
        user.remove();
        res.json({ message: 'User successfully deleted' });
      } else {
        res.status(403);
        res.json({ message: 'Could not find user.' });
        res.send();
      }
    }
  });
});

What I need to do is when a user is removed, his or her _id (String) should also be removed from all the other users' friends array. Hence the remove hook in the schema.

Right now the user gets deleted and the hook gets triggered, but the user _id is not removed from the friends array (tested with Postman):

[
    {
        "_id": "563155447e982194d02a4890",
        "username": "admin",
        "__v": 25,
        "password": "adminpass",
        "display_name": "admin",
        "friends": [
            "5633d1c02a8cd82f5c7c55d4"
        ]
    },
    {
        "_id": "5633d1c02a8cd82f5c7c55d4",
        "display_name": "Johnybruh",
        "password": "donttouchjohnsstuff",
        "username": "John stuff n things",
        "__v": 0,
        "friends": []
    }
]

To this:

[
    {
        "_id": "563155447e982194d02a4890",
        "username": "admin",
        "__v": 25,
        "password": "adminpass",
        "display_name": "admin",
        "friends": [
            "5633d1c02a8cd82f5c7c55d4"
        ]
    }
]

To try and figure it out I have looked at the Mongoosejs Documentation, but the mongoose doc example doesn't cover the remove hook. Also this stackoverflow qestion but this question seems to be about removing from other schemas.

I think i'm doing the remove in the hook wrong, but I can't seem to find the problem.

Thanks in advance!

EDIT:

I could not get the first suggestion by cmlndz to work, so I ended up fetching all the documents with arrays that contained the to-be-deleted users' id and pulling it from them one-by-one:

The delete function now contains this bit of code that does the magic:

// retrieve all documents that have this users' id in their friends lists
User.find({ friends: user._id }, function(err, friends) {
  if (err) {
    res.json({ warning: 'References not removed' });
  } else {

    // pull each reference to the deleted user one-by-one
    friends.forEach(function(friend){
      friend.friends.pull(user._id);
      friend.save(function(err) {
        if (err) {
          res.json({ warning: 'Not all references removed' });
        } 
      });
    });
  }
});

Answer 1

You could use $pull to find all documents that contain the "ID" in the "friends" array -or- find any matching document and popping the "ID" out of the array one by one.