There can only be one...

August 19, 2020

Use case: For a given database table with a table column storing boolean values, we only want to allow a single row to be true or false - for example, a Games table where only 1 game can be active at a time. We can use Active Record validations to constrain the truthy value to a single row.*

# == Schema Information
#
# Table name: games
#
#  id           :bigint
#  name         :string
#  description  :string
#  active       :boolean

class Game < ApplicationRecord
  validates_uniqueness_of :active, if: :active?
end

This is essentially saying, “If the value in the active column is true (if: :active?), then make sure it’s the only one (validates_uniquess_of)“.

It can also be expanded and scoped, i.e. “There can only be 1 active game at a venue”:

# == Schema Information
#
# Table name: game_venue
#
#  id        :bigint
#  game_id   :string
#  venue_id  :string
#  active    :boolean

class GameVenue < ApplicationRecord
  validates_uniqueness_of :active, scope: :venue_id, if: :active?
end

“For a given game at a particular venue, make sure it is the only active one”. Now, if we try to create two active GameVenue records, ActiveRecord will raise an error:

venue = Venue.first
game_venue_1 = GameVenue.create(venue_id: venue.id, active: true)
 # valid
game_venue_2 = GameVenue.create(venue_id: venue.id, active: true)
 # ActiveRecord::RecordInvalid - Active has already been taken