Eight years ago, Rails 5 made the belongs_to association required by default. This solved a lot of problems with orphaned records in the database. But sometimes, you need to have an optional association, which may lead to situations that you don’t expect.
First of all, yes, having an optional belongs_to brings denormalization to the database, but sometimes, it’s necessary. But besides denormalization, association validation can introduce bugs into your application. Let’s see how to validate an association in Rails. Let’s look at the example below.
# app/models/work/delivery.rb
class Work::Delivery < ApplicationRecord
# ...
belongs_to :inventory, optional: true
with_options if: -> { equipment? } do
validates :inventory_id, presence: true
end
# == Schema Information
#
# Table name: work_deliveries
#
# id :bigint not null, primary key
# type :string not null
# ppe_items :jsonb not null
# inventory_id :bigint
# created_at :datetime not null
# updated_at :datetime not null
#
# Indexes
#
# index_work_deliveries_on_inventory_id (inventory_id)
#
# Foreign Keys
# fk_rails_87309fb446 (inventory_id => inventories.id)
#
end
# app/models/inventory.rb
class Inventory < ApplicationRecord
has_many :work_deliveries, class_name: 'Work::Delivery'
# ...
end
In the example above, we have a Work::Delivery model that has an optional association with Inventory. But if the Work::Delivery is of type equipment the inventory should be present. To achieve this the author of the code used the with_options method to validate the inventory_id only if the equipment? method returns true.
Those, who start using Rails don’t see any problem with this code. Presence of the inventory_id is validated when needed. The problem is that the author validated presence of the model attribute, not the association. And this can lead to bugs in the application.
Let me give you an example.
# factories.rb
factory :inventory do
# ...
end
factory :work_delivery, class_name: "Work::Delivery" do
# ...
trait :equipment do
type { "equipment" }
inventory
end
end
The factory looks correct, let’s try to build the Work::Delivery with the :equipment trait.
delivery = FactoryBot.build(:work_delivery, :equipment)
delivery.valid? # => false
delivery.errors.full_messages # => ["Inventory must exist"]
delivery.inventory # => Inventory
The inventory association is created, but the delivery is not valid. The error message is Inventory must exist. But the inventory association is present. Wow, what’s going on?
The problem is that instead of validating the inventory association the code validates the inventory_id. And the inventory_id is not present.
Today you learned
When you validate associations in Rails – always validate the association, not the foreign key.
# app/models/work/delivery.rb
class Work::Delivery < ApplicationRecord
# ...
belongs_to :inventory, optional: true
with_options if: -> { equipment? } do
validates :inventory, presence: true
end
end
