Question

class User < ActiveRecord::Base
    has_many :boxes
    has_many :books
end

class Box < ActiveRecord::Base
    belong_to :user
    has_many :books
end

class Book < ActiveRecord::Base
    belongs_to :user
    belongs_to :box
end

So when I run below in a console it works as I expect, creating a new box and attaching the user_id to the box

>> u = User.first
>> u.boxes.build(:height => 3, :width => 1, :length => 4)
>> u.save

So then I go further and attempt this. The box_id is set, but the user_id in books is not set.

>> u.boxes.first.books.build(:title => 'Reading is fun')
>> u.save

It seems like I’m missing a pretty fundamental concept here.

Answer 1

Each relationship is distinct from all the others. That is, when you build a Book belonging to u.boxes.first, all Rails infers is that the Book belongs to that Box; it says nothing about the User.

In this situation, where (presumably) Books are always owned by the person who owns the Box they’re in, you’re probably best off with a has_many :through relationship:

class User < ActiveRecord::Base
    has_many :boxes
    has_many :books, :through => :boxes
end

class Box < ActiveRecord::Base
    belong_to :user
    has_many :books
end

class Book < ActiveRecord::Base        
    belongs_to: box
    delegate :user, :to => :box # since belongs_to doesn't take :through
                                # passes Book#user through to Book#box.user
end

If that’s not the case (perhaps you’re helping me move?), leave the associations as they are, but you’ll have to set at least one of the relations up manually, for instance:

b = u.boxes.first.books.build(:title => "foo")
b.user = u
b.save

# or...
u.boxes.first.create_book(:title => "foo", :user => u)

(Couple of other points. In your second example you should save the Book, not the User; and you can do both build and save in a single operation with create)