Validates Presence Of Foreign Key

When reading the Rails documentation for validates_presence_of I eyed the warning pertaining to foreign keys.

Rails Validation Warnings

I came across this article and when running the examples written, I found the same errors.

Stumped, I started crawling through the Rails code. I documented the behavior with this small rails app which includes the Rails 1.2.3 frozen in1.

Instead of using invoice and line_items, I decided to simply use parent and child

Parent:
  class Parent < ActiveRecord::Base
has_many :children
  end

Child:
  class Child < ActiveRecord::Base
belongs_to :parent
validates_presence_of :parent, :name
  end

Invalid Code:
  >> p = Parent.new(:name => "Dad")
  => #<Parent:0x23ac8f4 @attributes={"name"=>"Dad"}, @new_record=true>
  >> c = p.children.build(:name => "Son")
  => #<Child:0x2367858 @attributes={"name"=>"Son", "parent_id"=>nil}, @new_record=true>
  >> p.save!
  ActiveRecord::RecordInvalid: Validation failed: Children is invalid
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/validations.rb:764:in `save_without_transactions!'
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/transactions.rb:133:in `save!'
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:59:in `transaction'
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/transactions.rb:95:in `transaction'
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/transactions.rb:121:in `transaction'
      from ./script/../config/../config/../vendor/rails/activerecord/lib/active_record/transactions.rb:133:in `save!'
      from (irb):3

Valid Code:
  >> p = Parent.new(:name => "Dad")
  => #<Parent:0x22b9f28 @attributes={"name"=>"Dad"}, @new_record=true>
  >> c = p.children.build(:name => "Son", :parent => p)
  => #<Child:0x229626c @attributes={"name"=>"Son", "parent_id"=>nil}, @parent=#<Parent:0x22b9f28 @attributes={"name"=>"Dad"}, @new_record=true, @children=[#<Child:0x229626c ...>]>, @new_record=true>
  >> p.save!
  => true

What seems to be happening is the validations are called twice. The first time with the parent_id of nil, and then the second with the parent_id (obviously now it's saved the Parent and is going to actually save the Child).

Perhaps later I will crawl through the Rails code and submit a patch to automate the setting of association variable, but I'm still debating if that's the correct thing to do.

What gets really odd is if I take out the @validate_presence_of@ from the Child and use the first method, both Parent and Child get saved, and the Child has the foreign key set.

Parent:
  class Parent < ActiveRecord::Base
has_many :children
  end

Child:
  class Child < ActiveRecord::Base
belongs_to :parent
validates_presence_of :name
  end

Valid Code:
  >> p = Parent.new(:name => "Dad")
  => #<Parent:0x23ac8f4 @attributes={"name"=>"Dad"}, @new_record=true>
  >> c = p.children.build(:name => "Son")
  => #<Child:0x2368dfc @attributes={"name"=>"Son", "parent_id"=>nil}, @new_record=true>
  >> p.save!
  => true
  >> p.children
  => [#<Child:0x2368dfc @attributes={"name"=>"Son", "id"=>2, "parent_id"=>2}, @new_record=false, @errors=#<ActiveRecord::Errors:0x233fe84 @errors={}, @base=#<Child:0x2368dfc ...>>>]
1 To run the example: tar xvfz foreign_key_validations.tgz cd foreign_key_validations rake db:migrate rake test