Why does this model not show attributes which are accessible?

Question

I have two models, a Person and a Brain. Person has_one :brain, and Brain belongs_to :person. I want to assign Brain attributes through a /person/ update.

class Person < ActiveRecord::Base
  has_one :brain
  attr_accessible :name
  attr_accessible :brain
  accepts_nested_attributes_for :brain

end

class Brain < ActiveRecord::Base
  belongs_to :person
  attr_accessible :weight_kg

  attr_accessible :person
  accepts_nested_attributes_for :person
end

In the Rails console I can assign to Person.brain:

 > p = Person.first
=> #<Person id: 1, name: "Dave", created_at: "2013-02-14 20:17:35", updated_at: "2013-02-14 20:17:35"> 

 > p.brain.weight_kg = 5.0
  Brain Load (0.2ms)  SELECT "brains".* FROM "brains" WHERE "brains"."person_id" = 1 LIMIT 1
 => 5.0 
 > p.save
   (0.6ms)  begin transaction
   (0.6ms)  UPDATE "brains" SET "weight_kg" = 5.0, "updated_at" = '2013-02-14 20:18:11.010544' WHERE "brains"."id" = 1
   (317.6ms)  commit transaction
=> true 

Via the web form (and via the console) I cannot, because of the well-worn error, "Can't mass-assign protected attributes: brain_attributes".

I have attr_accessible :weight_kg in Brain, and in Person I have accepts_nested_attributes_for :brain, so I (wrongly) expect this to work.

What am I missing?

Answer 1

Change the attr_accessible to:

attr_accessible :brain_attributes