belongs_to through associations

can someone help me look at this. Getting too tired.

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 a 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. Thanks!

-------------Problems Reply------------

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.

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

Just use has_one instead of belongs_to in your :though, 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.

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.

You could simply use has_one in place of belongs_to :

has_one :question, :through => :answer

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

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.

Category:activerecord Views:0 Time:2010-10-26

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