mongodb node.js

I have a problem with this collection name:

module.exports = function() {

    var mongoose = require('mongoose');
    var db = mongoose.createConnection('localhost', 'race');
    db.on('error', console.error.bind(console, 'connection error:'));
    db.once('open', function () {});

    var collection = 'test';
    var Schema = mongoose.Schema;
    var ObjectId = Schema.ObjectId;

    var schema = new Schema({
        author: ObjectId,
        name: String,
        date: Date
    });

    this.model = db.model(collection, schema);

    var silence = new this.model({ name: 'Silence' })
    console.log(silence.name);
    silence.save();

    this.model.find(function (err, log) {
        console.log(err)
        console.log(log)
    })

    return this;
};  

I already have a test collection but console.log(log) just return Silence,

in fact Silence is register in "tests" collection and no "test"

can you explain me why, I set

var collection = 'test';