How to make associations work in built and unsaved objects?

Question

Let's say I have something like:

@order = Order.new(status: :pending)

@item  = @order.items.build(title: "Shirt")

When I try to call @item.order, I get an error not found, I guess it's because it's still unsaved to the DB, but how can make it point to the built object without saving any of the objects?

@item.order # Not Found - Tries to fetch from the DB

Answer 1

To make this work you have to tell the order association of your item model that it is the inverse of your items association or you order model and vice versa:

Like this:

class Item < ActiveRecord::Base
  belongs_to :order, inverse_of: :items
end

class Order < ActiveRecord::Base
  has_many :items, inverse_of: :order
end

That way your associations will be set up correctly even before saving to the database.

EDIT: If you don't want this you can always assign the association explicitly:

@item = @order.items.build(title: "Shirt")
@item.order = @order

or in one line:

@item = @order.items.build(title: "Shirt", order: @order)

Answer 2

I tested this in Rails 4.0.1 and it looks like using inverse_of is no longer needed as the associations are inferred properly. It was addressed in this commit: https://github.com/rails/rails/pull/9522

It is worth noting that there is still no solution for has_many through: relationships at this time.