Mongoose Populate in Node JS

Question

I've been trying to follow the information in Mongoose Population, but I'm getting the exception:

MissingSchemaError: Schema hasn't been registered for model "undefined".

The code I have goes like this:

mongoose = require('mongoose');
Schema = mongoose.Schema;
mongoose.connect(MONGO_SERVER);
ObjectId = Schema.ObjectId;

var FirstSchema = new Schema({
    label       : String
});
var SecondSchema = new Schema({
    first_id           : [{ type: mongoose.Schema.ObjectId, ref: 'First' }],
    type           : String,
    ...
});
var first= mongoose.model('First', FirstSchema);
var second= mongoose.model('Second', SecondSchema);

function test() {
    ...
    second.find({}).populate('first_id').exec(function(err,data){return true;});
    ...
}

And the error occurs on the populate, I've tweaked it a number of times to different answers found on forums, and I'm sure it will be something simple, but can someone point me in the right direction?

Cheers.

Answer 1

In your schema definitions, I see that you have defined 'first_id' as an array in Second schema. Compared to a relational database, this will be like an one-to-many relationship in which the parent table is the Second collection, and First collection as the child. Then you're doing wrong trying to populate the second with the first.

Suppose I have a Users collection, and a Clients collection, in which each client has an user related to it. Then the code will be:

var mongoose = require('mongoose');
mongoose.connect('mongodb://userName:password@server:port/dbname');
var conn = mongoose.connection;
conn.on('error', console.error.bind(console, 'connection error:'));
conn.once('open', function callback () {
    console.log('connected ');
});

var user = mongoose.Schema({
    userName: String
});

var client = mongoose.Schema({
    fk_user: { type: mongoose.Schema.ObjectId, ref: 'Users' },
    name: String
});

var UserModel = mongoose.model('Users', user);
var ClientModel = mongoose.model('Clients', client);

ClientModel.findOne().populate('fk_user').exec(function(err, c) {
    if (err) { return console.log(err); }

    console.log(c.fk_user.userName);
});

Hope this give you some point to help.