Rails has_one通过关系

Question

I have three tables: question, question_choices and response.

Question

• ID
• Name
• Tag

Question_Choice

• ID
• Question_id
• Label
• Value

Response

• ID
• User_id
• Question_id
• Value

Given a response, I want to see the question tag and the question_choice label for that. Getting either response or choice to relate to question are no problem, but getting response to relate to choice is proving challenging because I need to say "value=value". I have tried setting up a :has_one, :through => :question relationship but i'm not sure how to express the "value=value" stuff. Do I just need to use raw sql?

Answer 1

I assume that you don't want to change the table structure (which would be the easiest solution). This is the solution for your current table structure:

app/models/question.rb

class Question < ActiveRecord::Base
  has_many :question_choices
end

app/models/question_choice.rb

class QuestionChoice < ActiveRecord::Base
  belongs_to :question
end

app/models/response.rb

class Response < ActiveRecord::Base
  belongs_to :question
  belongs_to :question_choice, :primary_key => 'value', :foreign_key => 'value', :conditions => proc { "question_id = #{self.question_id}" }
end

Response.first.question_choice will give you the question_choice of the first response .

Answer 2

I'm guessing questions have many question choices, and that responses have a single 'selected' question choice. To model this you would need a direct association between the response and question choice as well, eg

class Question < ActiveRecord::Base
  has_many :question_choices
end

class QuestionChoice < ActiveRecord::Base
  belongs_to :question
end

class Response < ActiveRecord::Base
  belongs_to :user
  belongs_to :question
  belongs_to :question_choice
end

This means that you would need to add a question_choice_id column to your responses table.