Rails custom validation - Only one record can be true

Question

I'm trying to write a validation where only one record can be true. I have a 'game' model with an 'active' boolean column, only one game can be active at any time, so if someone tries to create a new 'game' record when there is an already active game then they should get an error. Below is what I currently have but isn't working!

validate :active_game

  def active_game
    if active == true && Game.find_by(active: true) == true
       errors[:name] = "a game is already active!"
    end
  end

Solution 1

You also need to check against the ID if the record is already persisted. Otherwise, saving the active game again would not succeed because there is an existing active game, which happens to be itself.

validate :only_one_active_game
scope :active, where(:active => true)

protected

def only_one_active_game
  return unless active?

  matches = Game.active
  if persisted?
    matches = matches.where('id != ?', id)
  end
  if matches.exists?
    errors.add(:active, 'cannot have another active game')
  end
end

Solution 2

I think you can just check the uniqueness of active_game when it is true.

validates_uniqueness_of :active_game, if: :active_game

Comments

[Tim S.]

What do you mean it isn't working?

[raphael_turtle]

I get an error from your code: PG::UndefinedFunction: ERROR: operator does not exist: integer <> LINE 1: ...petitions" WHERE "games"."active" = 't' AND (id != )

[lightswitch05]

He is missing a ?. 'id != ' should be 'id != ?'

[kristinalim]

Oops, sorry! You're right, I missed the question mark. Edited.

[kristinalim]

The id != ? part is problematic when id is NULL (i.e. not persisted). sqlfiddle.com/#!12/ae03e/3

[kristinalim]

Sorry that wasn't clear. SELECT * FROM books WHERE id != NULL AND active IS TRUE; returns a false negative (doesn't find the existing record) if there is already an active game. sqlfiddle.com/#!12/7d865/3 Not sure if this holds true for most DBMSes, but the standard for checking that there is a value is to use "IS NOT NULL". Correct me if I am wrong.