a "method_missing" example in ember.js. Catch setting undefined properties and serialize them to an "optionsString" property as a string of JSON.

user.js.coffee

App.User = DS.Model.extend
  name: DS.attr 'string'
  email: DS.attr 'string'
  # dynamically defined properties are serialized as a string of JSON here:
  optionsString: DS.attr 'string'

  # optionsData lets us deal with the string as a JSON object
  optionsData: ((k,v) ->
    if arguments.length == 2
      @set('optionsString', JSON.stringify(v))
      v
    else
      JSON.parse(@get('optionsString') || '{}')
  ).property('optionsString')

  # our dynamic "method_missing" getter
  unknownProperty: (k) ->
    @get('optionsData')[k]

  # our dynamic "method_missing" setter
  setUnknownProperty: (k, v) ->
    if k.match /(store|_reference)/ # careful not to store these objects!
      @[k] = v
    else
      o = @get('optionsData')
      o[k] = v
      @set('optionsData', o)
      o