.build can't assign foreign key that doesn't exist yet

Question

I have the following code for creating pictures for users in my users controller:

  def new
    @user = User.new
    @pictures = @user.pictures.build({:placement => "front"},{:placement => "profile"})
  end

However, when I create a new user, it isn't automatically building pictures with placement "front" or "profile." In fact, there's no update whatsoever to pictures. The pictures model has

attr_accessible :placement

and in picture.rb

belongs_to :user

and in user.rb

has_many :pictures
accepts_nested_attributes_for :pictures, allow_destroy: true

Why is my build command silently failing?

UPDATE

Using .save!, I have learned that @pictures are being assigned, but the problem is no user_id being associated with them. When I intentionally assign the user_id, as in:

def new
    @user = User.new
    @pictures = @user.pictures.build({:placement => "front", :user_id => @user.id},{:placement => "profile", :user_id => @user.id})
 end

It still does not assign a user_id.

Strangely, when I run the very same commands in the rails console, it does correctly assign the user_id.

It appears that the new @user does not get an auto id assigned to him until after the @pictures command is run, yet the console version is succeeding because it is performing the operations in sequence.

So, that's why I'm seeing a blank id.

Why is this happening? And isn't this simultaneous id assignment something the model is supposed to take care of with belongs_to and has_many , and accepts_nested_attributes_for ?

What is the appropriate way to address assigning nested attributes if the ID that links them together isn't created until after save?

Why does this not work?

Answer 1

Not sure if this is your problem, but you need to add :user_id to your attr_acessible line in picture.rb. Any attributes you want to read/write need to be attr_accessible or you will get that mass-assignment error.

Answer 2

It's in the docs: the user-ID is assigned by the database, after it is saved. So you have to save the user first, before you can use the user_id.

If you change :user_id => @user.id to :user => @user it should work, because then Rails is able to save them in the proper order, it knows it is not saved yet and needs to update those relations.