belongs to through associations

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   gt   answer end  class Answer   belongs to  question end  class Choice   belongs to  user   belongs to  answer   belongs to  question   through   gt   answer    validates uniqueness of  answer id   scope   gt     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   gt   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

User · Answer

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   gt   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

User · Answer

You can also delegate   class Company  lt  ActiveRecord  Base   has many  employees   has many  dogs   through   gt   employees end  class Employee  lt  ActiveRescord  Base   belongs to  company   has many  dogs end  class Dog  lt  ActiveRecord  Base   belongs to  employee    delegate  company   to   gt   employee   allow nil   gt  true end

User · 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   gt   Answer     validates inclusion of  choice   in   gt  lambda   answers   end  class Answer   belongs to  question end

User · Answer

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

User · Answer

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

User · Answer

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   gt     question id   user id           def question     answer question   end end   this way the you can now call question on Choice

User · Answer

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

[ruby-on-rails] belongs_to through associations

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