Rails - trouble using has_many relation

Question

I apologize if this is a big question. I used to be good at rails but it has been a long time.

I am working a rails project and am having trouble with the has_many relation.

I have the following tables:

User
SchoolClass
Question
UserClassQuestion

Inside the models i have:

user.rb
   has_many :questions, :through => :user_class_questions
   has_many :user_class_questions

school_class.rb
   has_many :questions, :through => :user_class_questions
   has_many :user_class_questions

question.rb
   belongs_to :schoolclass
   belongs_to :user

user_class_question.rb
   belongs_to :question

So, what I want is to on a users home page, display let them view the questions they have asked with current_user.questions . This works.

But on the SchoolClass show page, if i say @school_class.questions , i get the following error:

!!<ActiveRecord::StatementInvalid: SQLite3::SQLException: no such colum:
 user_class_questions.school_class_id: SELECT "questions".* FROM "questions" 
INNER JOIN "user_class_questions" ON "questions"."id" = 
"user_class_questions"."question_id" WHERE 
"user_class_questions"."school_class_id" = ?>

@school_class is an object set by the params

def set_school_class
   @school_class = SchoolClass.find(params[:id])
end

The columns in UserClassQuestion table are:

{ user_id: , schoolclass_id: , question_id }

So, do I have something set up wrong?

Answer 1

The error you're experiencing is most likely due to the variation in your column name. In Rails, school_class_id is not the same as schoolclass_id . I think a quick way to work around, is to specify the primary_key/foreign_key in your belongs_to option.

A more lasting solution is to run a migration to rename your column to school_class_id rather than schoolclass_id .

Let me know if I was able to help.

UPDATE

if the belongs_to :school_class is defined on question , then you have access to question.school_class and your school_class model should just have has_many :questions, no need of running to a join table to do that as that would confuse rails a bit. If however you don't want it this way you could comment out the belongs_to :school_class assoication and instead have a has_many :user_class_questions on question

class User
   has_many :questions, through: :user_class_questions
   has_many :user_class_questions
end

class SchoolClass
   has_many :questions, through: :user_class_questions
   has_many :user_class_questions
end

class Question
   #belongs_to :school_class
   belongs_to :user
   has_many :user_class_questions
end

class UserClassQuestion
   belongs_to :question
   belongs_to :school_class
   belongs_to :user
end

Answer 2

Your Data model needs a bit of a change, your UserClassQuestion is a join model and it should have data about the User, SchoolClass, and Question. So the relationship should be something like

UserClassQuestion rails g model UserClassQuestion user:references school_class:references question:references

class UserClassQuestion < ActiveRecord::Base
  belongs_to :user
  belongs_to :school_class
  belongs_to :question
end

school_class.rb

class SchoolClass < ActiveRecord::Base
  has_many :user_class_questions
  has_many :questions, through: :user_class_questions
end

user.rb

class User < ActiveRecord::Base
  has_many :user_class_questions
  has_many :questions, through: :user_class_questions
end

And then you can do user.questions and school_class.questions

Also try not to use has_many_and_belongs_to association, I feel it is a bad idea, it creates a table without a primary id, and later on if you want to do something else with this join table, then this will come and bite you. has_many, through is a bit extra step but it is worth it.