How to get the results of other tables using includes or joins in Ruby on Rails

Question

I want to query the results of private_classes. The scenario is:

class PrivateClass < ActiveRecord::Base
  belongs_to :private_school
  has_many :lesson_plans
end

class JoinedPrivateSchool < ActiveRecord::Base
  belongs_to :student
  belongs_to :private_school
end

class PrivateSchool < ActiveRecord::Base
  belongs_to :teacher
  has_many :private_classes
  has_many :joined_private_schools
end

class Student < ActiveRecord::Base
  has_many :joined_private_schools
end

JoinPrivateSchool has the attribute private_school_id.

I am doing:

s = Student.find(9)

s.joined_private_schools

which results in:

=> #<ActiveRecord::Associations::CollectionProxy [#<JoinedPrivateSchool id: 8,
 user_id: 73, student_id: 9, private_school_id: 28, created_at: "2017-02-16 
12:38:37", updated_at: "2017-02-16 12:38:37">, #<JoinedPrivateSchool id: 9,
 user_id: 73, student_id: 9, private_school_id: 33, created_at: "2017-02-16 
12:42:01", updated_at: "2017-02-16 12:42:01">, #<JoinedPrivateSchool id: 12, 
user_id: 73, student_id: 9, private_school_id: 32, created_at: "2017-02-16 
13:19:02", updated_at: "2017-02-16 13:19:02">]>

If I do:

c = s.joined_private_schools.includes(private_school: :private_classes)

it results in:

JoinedPrivateSchool Load (0.6ms)  SELECT "joined_private_schools".* FROM 
"joined_private_schools" WHERE "joined_private_schools"."student_id" = $1 
 [["student_id", 9]]


PrivateSchool Load (0.9ms)  SELECT "private_schools".* FROM "private_schools" 
WHERE "private_schools"."id" IN (28, 33, 32)  ORDER BY 
"private_schools"."created_at" DESC


PrivateClass Load (0.7ms)  SELECT "private_classes".* FROM "private_classes" 
WHERE "private_classes"."private_school_id" IN (33, 32, 28)  ORDER BY 
"private_classes"."created_at" DESC


=> #<ActiveRecord::AssociationRelation [#<JoinedPrivateSchool id: 8, user_id: 
73, student_id: 9, private_school_id: 28, created_at: "2017-02-16 12:38:37",
 updated_at: "2017-02-16 12:38:37">, #<JoinedPrivateSchool id: 9, user_id: 73,
 student_id: 9, private_school_id: 33, created_at: "2017-02-16 12:42:01", 
updated_at: "2017-02-16 12:42:01">, #<JoinedPrivateSchool id: 12, user_id: 73,
 student_id: 9, private_school_id: 32, created_at: "2017-02-16 13:19:02", 
updated_at: "2017-02-16 13:19:02">]> 

But that is still the wrong results.

I need to get the results of private_classes using one query so I can avoid multiple each loops.

Answer 1

If you want to have PrivateClass, you're starting from the wrong point (User). You need to start from PrivateClass joining all the intermediary tables and using the user id as condition:

PrivateClass.joins(private_school: :user).where(user: {id: 9})

# or a little more performant (avoids one join)

PrivateClass.joins(:private_school).where(private_school: {user_id: 9})

Answer 2

You can use :through on associations (at least in Rails 4.2)
I updated this, I first missed that you wanted private_classes

class JoinedPrivateSchool < ActiveRecord::Base
  belongs_to :student
  belongs_to :private_school
  has_many :private_classes, through: :private_schools
end

class Student < ActiveRecord::Base
  has_many :joined_private_schools
  has_many :private_classes, through: :joined_private_schools
end

s = Student.find(9)
s.private_classes