如何验证 has_many :through 连接模型的唯一性？

我有通过投票模型加入的用户和问题。用户可以对问题进行投票。他们可以投票赞成或反对（这记录在投票模型中）。首先，我希望能够阻止用户向一个方向投多次票。其次，我想让用户投反对票。因此，如果他们投了赞成票，他们仍然应该能够投反对票，这将取代赞成票。用户永远不能对一个问题进行两次投票。这是我的文件：

class Issue < ActiveRecord::Base
  has_many :associations, :dependent => :destroy

  has_many :users, :through => :associations

  has_many :voterships, :dependent => :destroy
  has_many :users, :through => :voterships

  belongs_to :app

  STATUS = ['Open', 'Closed']

  validates :subject, :presence => true,
                      :length => { :maximum => 50 }
  validates :description, :presence => true,
                          :length => { :maximum => 200 }
  validates :type, :presence => true
  validates :status, :presence => true

  def cast_vote_up!(user_id, direction)
    voterships.create!(:issue_id => self.id, :user_id   => user_id,
                                             :direction => direction)
  end
end


class Votership < ActiveRecord::Base
  belongs_to :user
  belongs_to :issue
end

class VotershipsController < ApplicationController
  def create
    session[:return_to] = request.referrer
    @issue = Issue.find(params[:votership][:issue_id])
    @issue.cast_vote_up!(current_user.id, "up")
    redirect_to session[:return_to]
  end
end

class User < ActiveRecord::Base
  authenticates_with_sorcery!

  attr_accessible :email, :password, :password_confirmation

  validates_confirmation_of :password
  validates_presence_of :password, :on => :create
  validates_presence_of :email
  validates_uniqueness_of :email

  has_many :associations, :dependent => :destroy
  has_many :issues, :through => :associations

  has_many :voterships, :dependent => :destroy
  has_many :issues, :through => :voterships
end

您可以将唯一性约束放在Votership模型。您不需要对关联本身进行验证。

class Votership < ActiveRecord::Base
  belongs_to :user
  belongs_to :issue

  validates :issue_id, :uniqueness => {:scope=>:user_id}
end

这意味着用户只能对给定问题进行单次投票（赞成或反对）。