Rails ActiveRecord Database with just one row

Question

Is there a way for a database to only allow one row (e.g. for site-wide settings) ?

Answer 1

class Whatever < ActiveRecord::Base
  validate :there_can_only_be_one

  private

  def there_can_only_be_one
    errors.add_to_base('There can only be one') if Whatever.count > 0
  end
end

Answer 2

In Rails 4:

class Anything < ActiveRecord::Base
  before_create :only_one_row

  private
  def only_one_row
    false if Anything.count > 0
  end
end

Silent errors are bad, then

class Anything < ActiveRecord::Base
  before_create :only_one_row

  private
  def only_one_row
    raise "You can create only one row of this table" if Anything.count > 0
  end
end