A glimpse at how I'm doing validations.

models.coffee

define ['backbone', 'validate/validators'], (Backbone, Validators) ->

  Validators.extend Backbone.Model

  Child: class Child extends Backbone.Model
    defaults:
      name:''
      email:''
      invited:false

    @required 'name'
    @required 'email', when:'invited'

  Coparent: class Coparent extends Backbone.Model
    defaults:
      name:''
      email:''
      invited:false
      timezone:''

    @required 'name', 'timezone'
    @required 'email', when:'invited'

  Cofamily: class Cofamily extends Backbone.Model
    initialize: ->
      @children = new Backbone.Collection
      @primaryCoparent = new Coparent
      @coparent = new Coparent

    addChild: (attrs) ->
      @children.add new Child(attrs)

The validators are declared in the models, but all the work is done with a ModelFormValidator, which knows how to use the validators to validate the values of visible HTML form fields using Validator#validate(model, value). Validator knows the attribute it's responsible for and has options like when:.