Rails ActiveRecord数据库只有一行

Question

有没有办法使数据库只允许一行(例如,对于站点范围的设置)？

Answer 1

class Whatever < ActiveRecord::Base
  validates :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

在 Rails 4 中：

class Anything < ActiveRecord::Base
  before_create :only_one_row

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

无声的错误是不好的，那么

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