Removing many to many reference in Mongoose

Question

One of my mongoose schemas is a many to many relationship:

var UserSchema = new Schema({
   name       : String,
   groups  : [ {type : mongoose.Schema.ObjectId, ref : 'Group'} ]
});

var GroupSchema = new Schema({
   name       : String,
   users  : [ {type : mongoose.Schema.ObjectId, ref : 'User'} ]
});

If I remove a group, is there anyway to remove that group objectId from all the user's 'groups' array?

GroupSchema.pre('remove', function(next){
    //Remove group._id from all the users
})

Answer 1

You're on the right track to use 'remove' middleware for this. In the middleware function, this is the group instance being removed and you can access the other models via its model method. So you can do something like:

GroupSchema.pre('remove', function(next){
    this.model('User').update(
        {_id: {$in: this.users}}, 
        {$pull: {groups: this._id}}, 
        {multi: true},
        next
    );
});

Or if you want to support cases where the users field in your group instance may not be complete you could do:

GroupSchema.pre('remove', function(next){
    this.model('User').update(
        {groups: this._id}, 
        {$pull: {groups: this._id}}, 
        {multi: true},
        next
    );
});

But as WiredPrairie notes, for this option you'd want an index on groups for good performance.

Answer 2

I use my patched version of mongoose-relationship "plugin" to solve this: take a look on https://github.com/begrossi/mongoose-relationship/tree/remove-from-parent-if-removed-from-child-set.

Bruno Grossi