belongs_to through associations - ruby-on-rails

Question

Given the following associations, I need to reference the Question that a Choice is attached through from the Choice model. I have been attempting to use belongs_to :question, through: :answer to perform this action.
class User
has_many :questions
has_many :choices
end
class Question
belongs_to :user
has_many :answers
has_one :choice, :through => :answer
end
class Answer
belongs_to :question
end
class Choice
belongs_to :user
belongs_to :answer
belongs_to :question, :through => :answer
validates_uniqueness_of :answer_id, :scope => [ :question_id, :user_id ]
end
I am getting
NameError uninitialized constant User::Choice
when I try to do current_user.choices
It works fine, if I don't include the
belongs_to :question, :through => :answer
But I want to use that because I want to be able to do the validates_uniqueness_of
I am probably overlooking something simple. Any help would be appreciated.

Answer 1

You can also delegate:
class Company < ActiveRecord::Base
has_many :employees
has_many :dogs, :through => :employees
end
class Employee < ActiveRescord::Base
belongs_to :company
has_many :dogs
end
class Dog < ActiveRecord::Base
belongs_to :employee
delegate :company, :to => :employee, :allow_nil => true
end

Answer 2

Just use has_one instead of belongs_to in your :through, like this:
class Choice
belongs_to :user
belongs_to :answer
has_one :question, :through => :answer
end
Unrelated, but I'd be hesitant to use validates_uniqueness_of instead of using a proper unique constraint in your database. When you do this in ruby you have race conditions.

Answer 3

A belongs_to association cannot have a :through option. You're better off caching the question_id on Choice and adding a unique index to the table (especially because validates_uniqueness_of is prone to race conditions).
If you're paranoid, add a custom validation to Choice that confirms that the answer's question_id matches, but it sounds like the end user should never be given the opportunity to submit data that would create this kind of mismatch.

Answer 4

My approach was to make a virtual attribute instead of adding database columns.
class Choice
belongs_to :user
belongs_to :answer
# ------- Helpers -------
def question
answer.question
end
# extra sugar
def question_id
answer.question_id
end
end
This approach is pretty simple, but comes with tradeoffs. It requires Rails to load answer from the db, and then question. This can be optimized later by eager loading the associations you need (i.e. c = Choice.first(include: {answer: :question})), however, if this optimization is necessary, then stephencelis' answer is probably a better performance decision.
There's a time and place for certain choices, and I think this choice is better when prototyping. I wouldn't use it for production code unless I knew it was for an infrequent use case.

Answer 5

So you cant have the behavior that you want but you can do something that feels like it. You want to be able to do Choice.first.question
what I have done in the past is something like this
class Choice
belongs_to :user
belongs_to :answer
validates_uniqueness_of :answer_id, :scope => [ :question_id, :user_id ]
...
def question
answer.question
end
end
this way the you can now call question on Choice

Answer 6

It sounds like what you want is a User who has many Questions.
The Question has many Answers, one of which is the User's Choice.
Is this what you are after?
I would model something like that along these lines:
class User
has_many :questions
end
class Question
belongs_to :user
has_many :answers
has_one :choice, :class_name => "Answer"
validates_inclusion_of :choice, :in => lambda { answers }
end
class Answer
belongs_to :question
end

Answer 7

The has_many :choices creates an association named choices, not choice. Try using current_user.choices instead.
See the ActiveRecord::Associations documentation for information about about the has_many magic.