Replacing embedded document array with a string array in MongoDB

Question

In my collection, I have a users field as an array of the User collection.

So, it currently looks like this:

{ "name" : "Untitled", "users" : [  {   "name" : "Ace Ventura",     "datecreated" : "2012-10-05T23:55:56.940Z",     "_id" : "740063fb-79c5-4f7f-96e1-907d6ffb1d16" } ], "datecreated" : "2012-10-05T23:55:56.954Z", "_id" : "e207eaea-89f7-48ae-8ba7-b6aa39db2358" }

I'd like to update it so that the array of User collection becomes just an array of the _id property of the User collection. Like so:

{ "name" : "Untitled", "users" : ["740063fb-79c5-4f7f-96e1-907d6ffb1d16" ], "datecreated" : "2012-10-05T23:55:56.954Z", "_id" : "e207eaea-89f7-48ae-8ba7-b6aa39db2358" }

How would I accomplish this?

Thanks in advance.

Answer 1

Alright, I figured it out.

Initially, I was thinking of doing something like this:

db.lists.update(
   {},
   {
     $set: {
             users: <oldusers._id>
   },
});

However, as it turns out, you cannot reference the current document's properties from within an update() .

But, it turns out we can use forEach() :

db.lists.find().forEach(function (list) {

    // Define length to ensure we have an array of users
    var userLength  = (list.users && list.users.length) || 0;

    var (var i = 0; i < userLength; i++) {
        // Ensure the current user object isn't null or undefined
        if(list.users[i])
            list.users[i] = list.users[i]._id
    }

    // Finally, save the list object back.
    db.lists.save(list)
});

Thank, +gipset, for your answer here .