JS model and collection classes

/**
 * Model class
 */
Model = function(url, attributes) {
  this.url = url;
  this.attributes = attributes;
};

Model.prototype = {
  set: function(data) {
    this.attributes = data;
  },
  fetch: function() {
    return $.getJSON(this.url + '/' + this.attributes.id, this.set.bind(this));
  },
  save: function() {
    return $.ajax({
      type: 'PUT',
      contentType: 'application/json',
      url: this.url,
      data: JSON.stringify(this.attributes),
      success: this.set.bind(this)
    });
  }
};


/**
 * Collection class
 */
Collection = function(url) {
  this.url = url;
};

Collection.prototype = {
  _expand: function(data) {
    this.data = [];
    for (var i = 0; i < data.length; i++) {
      this.data.push(new Model(this.url, data[i]));
    }
  },
  fetch: function() {
    return $.getJSON(this.url, this._expand.bind(this))
  },
  _flatten: function() {
    var ret = [];
    for (var i = 0; i < this.data.length; i++) {
      ret.push(this.data[i].attributes);
    }
    return ret;
  },
  save: function() {
    return $.ajax({
      type: 'PUT',
      url: this.url,
      contentType: 'application/json',
      data: JSON.stringify(this._flatten),
      success: this.set.bind(this)
    });
  }
};