CODEWITHSUNDEEP
ruby-on-railsdatabaseactiverecord
pitosalas
pitosalas

Reputation: 10902

Multi-target has_many through:

I have three models:

User(name: string)
Course(name: string)
Assoc(user_id: integer, ta_id: integer, teach_id: integer, student_id: integer)

I want to be able to associate a user with a course, as a ta, a teacher, or a student. I am not sure this matches the :polymorphic idea in ActiveRecord. Here's what I have so far, but it's not quite working:

class User < ApplicationRecord
  has_many :tas, class_name: "Assoc", foreign_key: :ta_id
  has_many :teaches, class_name: "Assoc", foreign_key: :teach_id
  has_many :takes, class_name: "Assoc", foreign_key: :student_id

  has_many :ta_courses, through: :tas
  has_many :taken_courses, through: :tas
  has_many :taught_courses, through: :tas
end

It's not working: 

irb(main):056:0> User.find(1).ta_courses
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
Traceback (most recent call last):
        1: from (irb):56
ActiveRecord::HasManyThroughSourceAssociationNotFoundError (Could not find the source association(s) "ta_course" or :ta_courses in model Assoc. Try 'has_many :ta_courses, :through => :tas, :source => <name>'. Is it one of ?)
irb(main):057:0>

Any pointers would be appreciated!

Upvotes: 1

Views: 166

Answers (2)

max
max

Reputation: 102343

If I really had to be able to query all the user -> course relations as a single table I would set it up as so:

# rails g model assoc user:belongs_to course:belongs_to
class Assoc < ApplicationRecord
  enum role: [:student, :teacher, :assistent]
  belongs_to :user
  belongs_to :course
end

We then want to setup scoped assocations in User and Course for each role:

has_many :student_assocs, -> { where(role: :student) }
  class_name: 'Assoc'

Since we want to have the exact same assocations in both lets keep it DRY by using a module:

app/models/concerns/associated.rb
# dynamically creates assocations for each role in Assoc.roles enum
module Associated
  extend ActiveSupport::Concern
  included do
    # the plain base assocation
    has_many :assocs
    # this creates the assocations student_assocs, teacher_assocs, etc
    Assoc.roles.keys.each do |role|
      # We need to use eval for the second argument as we are creating the lambda dynamically
      has_many :"#{role}_assocs", eval( "->{ where(role: #{Assoc.roles[role]})}" ),
        class_name: 'Assoc'
    end
  end
end

Assoc.roles gives a hash of the enum mappings we set up in Assoc.

We can then include our module in Course and User and setup the indirect assocations:

class Course < ApplicationRecord
  include Associated
  has_many :users, through: :assocs
  # this creates the assocations students, teachers, etc
  Assoc.roles.keys.each do |role|
    has_many role.pluralize.to_sym,
      through: "#{role}_assocs".to_sym,
      source: :user
  end
end

class User < ApplicationRecord
  include Associated
  has_many :courses, through: :assocs
   # this creates the assocations courses_as_student, courses_as_teacher, etc
  Assoc.roles.keys.each do |role|
    has_many "course_as_#{role}".to_sym,
      through: "#{role}_assocs".to_sym,
      source: :course
  end
end

Upvotes: 1

Mark
Mark

Reputation: 71

It looks like you haven't built the relationships from the Assoc model to the Course model. Your Assoc model should have a course_id so it would look like:

Assoc(user_id: integer, ta_id: integer, teach_id: integer, student_id: integer, course_id: integer)

Then you would need a belongs_to relationship on the Assoc model:

class Assoc < ApplicationRecord
  belongs_to :course
end

Finally, your has_many through relationships in the User model aren't built correctly (all are through tas). Your error message gives you a clue as to the last thing you need to do, which is identify the aliased relationship using source:

class User < ApplicationRecord
  has_many :tas, class_name: "Assoc", foreign_key: :ta_id
  has_many :teaches, class_name: "Assoc", foreign_key: :teach_id
  has_many :takes, class_name: "Assoc", foreign_key: :student_id

  has_many :ta_courses, through: :tas, source: :course
  has_many :taught_courses, through: :teaches, source: :course
  has_many :taken_courses, through: :takes, source: :course
end

Upvotes: 1

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