Populate after find()

[Stefano Passiatore]

Hi, I've these Schemas:

var InputSchema = new mongoose.Schema({
    url: String,
    titleSource: String,
    urls: [{
        type: mongoose.Schema.Types.ObjectId,
        ref: 'Url'
    }]
});


var UrlSchema = new mongoose.Schema({
    description: String,
    url: String,
    _source: {
        type: String,
        ref: 'Input'
    }
});


but this code return undefined... and only one time (I've several instances of Urls)
    Url.find()
    .populate('_source', 'titleSource') // only return the Persons name
    .exec(function (err, url) {
        if (err) return console.log(err);
        console.log(url._source);  
    })

Why? Thanks a lot!

Stefano