data not stored node.js mongodb

I have a problem to store data in mongoDb,

This is my model:

module.exports = function(app) {

    var collection = 'chat_user';
    var Schema = app.mongoose.Schema;

    var schema = new Schema({
        user_id: String,
        ws: Object
    }, { collection: collection });

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

    return this;
};

But when I'm trying to store :

chat.user = new models.chat_user({
    user_id: app.locals.session.user._id,
    ws: ws
})
console.log(chat.user)
chat.user.save();

The console.log return the good result but I have nothing in my database, maybe the object si too big ?

If I remove it my object_id is store in my db

Thanks