foo.coffee

MyAdapter = Ember.RESTAdapter.extend
  didSaveRecord: (record, data) ->
    rootKey = Em.get(record.constructor, 'rootKey')
    primaryKey = Em.get(record.constructor, 'primaryKey')
    dataToLoad = if rootKey then data[rootKey] else data
    if dataToLoad
      record.load(dataToLoad[primaryKey], dataToLoad)
    record.didSaveRecord()
    
App.Book.adapter = MyAdapter.create()

server.rb

# On server

def update
  book = Book.find params[:id]
  if book.update_attributes params
    render json: book
  else
    raise "didn't save book"
  end
end