Adding JOIN for associated tables [duplicate] - ruby-on-rails

Question

This question already has an answer here:
Adding a JOIN between two tables
(1 answer)
Closed 9 years ago.
My Organization class looks something like this:
has_many Students
My Student class looks like this:
has_many Klasses
belongs_to Organization
My Klass class looks like this:
some field named : price
scope :top_expensive_classes, joins(:students).order('price DESC')
belongs_to Student
And my query looks like this:
#results = Klass.top_expensive_classes.where(organization_id: params[:id]).limit(RESULT_SET_COUNT)
Notice that it starts with Klass, so that's the problem because I am searching in the where class for organization_id but that is not in the Klass, it is in Student class , so somehow I should introduce a join somewhere to fix this but couldn't figure it out.

Answer 1

I think the real issue is your associations are likely incorrect.
A student has many classes
A class has many students
but what you have is
A student has many classes
A class belongs to a single student
This doesn't really make sense (at least in any situation I've ever seen a class and student interact). You should be creating a many-to-many relationship instead of a one-to-many relationship between Klass and Student.
class Student < ActiveRecord::Base
has_many :klasses, through: :student_klasses
has_many :student_klasses
end
class Klass < ActiveRecord::Base
has_many :students, through: :student_klasses
has_many :student_klasses
end
class StudentKlass < ActiveRecord::Base
belongs_to :student
belongs_to :klass
end
Once you have these correct associations in place, you need to call .joins on the :students association from the Klass class. You can do without the scope.
Klass.joins(:students).where("students.organization_id = ?", params[:id]).order('price DESC').limit(RESULT_SET_COUNT)
Read the guide on ActiveRecord Querying.
Here is the proof (using the exact model definitions above) that the ordering of the associations does not matter.
irb(main):001:0> s = Student.create(name: "Deefour")
SQL (3.6ms) INSERT INTO "students" ("created_at", "name", "updated_at") VALUES (?, ?, ?) [["created_at", Fri, 08 Mar 2013 01:33:32 UTC +00:00], ["name", "Deefour"], ["updated_at", Fri, 08 Mar 2013 01:33:32 UTC +00:00]]
=> #<Student id: 1, name: "Deefour", created_at: "2013-03-08 01:33:32", updated_at: "2013-03-08 01:33:32">
irb(main):002:0> kk = []
=> []
irb(main):003:0> kk << Klass.create(title: "Klass 1")
SQL (0.3ms) INSERT INTO "klasses" ("created_at", "title", "updated_at") VALUES (?, ?, ?) [["created_at", Fri, 08 Mar 2013 01:34:06 UTC +00:00], ["title", "Klass 1"], ["updated_at", Fri, 08 Mar 2013 01:34:06 UTC +00:00]]
=> [#<Klass id: 1, title: "Klass 1", created_at: "2013-03-08 01:34:06", updated_at: "2013-03-08 01:34:06">]
irb(main):004:0> kk << Klass.create(title: "Klass 2")
SQL (0.3ms) INSERT INTO "klasses" ("created_at", "title", "updated_at") VALUES (?, ?, ?) [["created_at", Fri, 08 Mar 2013 01:34:14 UTC +00:00], ["title", "Klass 2"], ["updated_at", Fri, 08 Mar 2013 01:34:14 UTC +00:00]]
=> [#<Klass id: 1, title: "Klass 1", created_at: "2013-03-08 01:34:06", updated_at: "2013-03-08 01:34:06">, #<Klass id: 2, title: "Klass 2", created_at: "2013-03-08 01:34:14", updated_at: "2013-03-08 01:34:14">]
irb(main):005:0> s.klasses = kk
Klass Load (0.1ms) SELECT "klasses".* FROM "klasses" INNER JOIN "student_klasses" ON "klasses"."id" = "student_klasses"."klass_id" WHERE "student_klasses"."student_id" = ? [["student_id", 1]]
SQL (0.4ms) INSERT INTO "student_klasses" ("created_at", "klass_id", "student_id", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Fri, 08 Mar 2013 01:34:29 UTC +00:00], ["klass_id", 1], ["student_id", 1], ["updated_at", Fri, 08 Mar 2013 01:34:29 UTC +00:00]]
SQL (0.1ms) INSERT INTO "student_klasses" ("created_at", "klass_id", "student_id", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Fri, 08 Mar 2013 01:34:29 UTC +00:00], ["klass_id", 2], ["student_id", 1], ["updated_at", Fri, 08 Mar 2013 01:34:29 UTC +00:00]]
=> [#<Klass id: 1, title: "Klass 1", created_at: "2013-03-08 01:34:06", updated_at: "2013-03-08 01:34:06">, #<Klass id: 2, title: "Klass 2", created_at: "2013-03-08 01:34:14", updated_at: "2013-03-08 01:34:14">]
irb(main):006:0> Student.first.klasses.map(&:id)
Student Load (0.2ms) SELECT "students".* FROM "students" ORDER BY "students"."id" ASC LIMIT 1
Klass Load (0.1ms) SELECT "klasses".* FROM "klasses" INNER JOIN "student_klasses" ON "klasses"."id" = "student_klasses"."klass_id" WHERE "student_klasses"."student_id" = ? [["student_id", 1]]
=> [1, 2]