Unique on three different fields

I am working on a Ruby on Rails web application. I want to validate the uniqueness of more than one field together. How can i do this?

For example: I have a model named waiting with t开发者_运维百科hree fields:

project_id category_id and user_id

I want to ensure that i won't have two identical rows in all three fields.

Why not just to:

validates_uniqueness_of :user_id, :scope => [:project_id, :category_id]

+

add_index :waitings, [:project_id, :category_id, :user_id], :unique => true

Read API:

• http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of
• http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index

It's not pretty but this works for me:

class Waiting < ActiveRecord::Base
  validate :must_be_unique

  def must_be_unique
    if self.class.where(project_id: project_id, category_id: category_id, user_id: user_id).exists?
      errors.add(:base, 'Must be unique')
    end
  end
end

Of course you could just use a unique key in your db schema and then catch the relevant exceptions on the rare occasion you need to.