Rails custom validation - Only one record can be true

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

validates_uniqueness_of :active_game, if: :active_game

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