base_model.js

App.Models.BaseModel = Backbone.Model.extend({

  preParse: function(data){},

  constructor: function(attributes, options){
    options = options || {};
    options.parse = true;
    Backbone.Model.call(this, attributes, options);
  },

  wrapAttribute: function(attributes, key, backboneType) {
    var value = attributes[key];
    if (attributes[key] && isNotTypeOf(backboneType, value)) {
      attributes[key] = new backboneType(value);
    }
  },

  parse: function(data, options){
    this.preParse(data);
    return data;
  }

});

App.Models.Team = App.Models.BaseModel.extend({

  preParse: function(data){
    this.wrapAttribute(data,'roster',App.Collections.Players);
  }

});

App.Models.Player = App.Models.BaseModel.extend({});

App.Collections.Players = Backbone.Collection.extend({
  model: App.Models.Player
});

base_model_example.js

var team = new App.Models.Team({
  name: "Bulls",
  city: "Chicago",
  roster: [{name: "Derrick Rose"}, {name: "Joakim Noah"}]
});

var player1 = team.get('roster').first();
player1.get('name'); // => "Derrick Rose"
player1.set('name', "D. Rose");
player1.save();

defaults_with_object.js

App.Models.Team = Backbone.Model.extend({
  defaults: {
    roster: new App.Collections.Players
  }
});

defaults_with_object_in_function.js

App.Models.Team = Backbone.Model.extend({

  defaults: function(){
     return {roster: new App.Collections.Players};
  }

});

var team1 = new App.Models.Team;
var team2 = new App.Models.Team;
team1.get('roster').add(new App.Models.Player);
team1.get('roster').size(); // => 1, That's fine
team2.get('roster').size(); // => 0, Yes!
(new App.Models.Team).get('roster').size(); // => 0 Expected!

defaults_with_object_usage.js

var team1 = new App.Models.Team;
var team2 = new App.Models.Team;
team1.get('roster').add(new App.Models.Player);
team1.get('roster').size(); // => 1, That's fine
team2.get('roster').size(); // => 1, That's totally not fine
(new App.Models.Team).get('roster').size(); // => 1, That's just weird

user.js

// In app/assets/javascripts/models/user.js

App.Models.User = Backbone.Model.extend({
  urlRoot: '/url-goes-here'
});

App.Collections.Users = Backbone.Collection.extend({
  url: '/url-goes-here',
  model: App.Models.User
});