bad.js

var MyModel = Backbone.Model.extend({
  urlRoot: '/wibble',
  clone: function(){
    var attrs = _.chain(this.attributes).omit('id', 'cid').clone().value()
    return new this.constructor(attrs)
  }
})

var m1 = new MyModel()

m1.save({ 
  foo: { bar: { quux: 'bizzle' } }
})

var m2 = m1.clone()

m2.get('foo').bar = 'baz'
m1.get('foo').bar // 'baz' - d'oh ;-;

better.js

var MyModel = Backbone.Model.extend({
  urlRoot: '/wibble',
  clone: function(){
    var attrs = _.chain(this.attributes).omit('id', 'cid').cloneDeep().value()
    return new this.constructor(attrs)
  }
})

var m1 = new MyModel()

m1.save({ 
  foo: { bar: { quux: 'bizzle' } }
})

var m2 = m1.clone()

m2.get('foo').bar = 'baz'
m1.get('foo').bar // { quux: 'bizzle' }  - hurrah