Using SchemaTypes other than ObjectId for Population (Mongoose)

Question

I am relatively new to node.js and mongoose, so I've been trying to get population working for mongoose. According to the official mongoose documentation, Strings can be used as refs:

Note: ObjectId, Number, String, and Buffer are valid for use as refs.

Based on this, I've been trying to populate a field that is of type String, rather than ObjectId. However, every time I try to do so, I get a CastError:

CastError: Cast to ObjectId failed for value "testuser" at path "_id"

If it helps, here's my code:

/=============== In user.js ===============/
var UserSchema = new mongoose.Schema({
    _username: { type: String, required: true, unique: true },
    password: { type: String, required: true },
    date_created: { type: Date, required: true, default: Date.now() },
    last_accessed: { type: Date, required: true },
    admin_access: { type: Boolean, required: true } 
});

UserSchema.post('save', function(user) {
    // Settings Model
    var Settings = require('./settings');
    var setting = new Settings.SettingsModel({
        _user: user._username
    });

    setting.save(function(err) {
        if (err) throw err;
        console.log('Successfully created settings for user: ' + setting);

        Settings.SettingsModel.findOne({ _user: user._username }, function(err, setting) {
            Settings.SettingsModel.populate(setting, { path: '_user' }, function(err, setting) {
                if (err) throw err;
                console.log('User of setting is ' + setting._user);
            });
        });
    });
});

/=============== In settings.js ===============/
var SettingsSchema = new mongoose.Schema({
    _user: { type: String, ref: 'User', unique: true },
    defaultCurrency: { type: String, default: 'USD', required: true },
    secondaryCurrency: { type: String }
});

Basically, settings for a user is created after a user has been created.

Any help is appreciated. Thanks!

Answer 1

While you can use types other than ObjectId for refs, the refs can't reference any field besides _id. So to do this you'd have to declare _id of UserSchema as a String and set its value to the same value you give username:

var UserSchema = new mongoose.Schema({
    _id: { type: String },
    _username: { type: String, required: true, unique: true },
    password: { type: String, required: true },
    date_created: { type: Date, required: true, default: Date.now() },
    last_accessed: { type: Date, required: true },
    admin_access: { type: Boolean, required: true } 
});