如何使用包括或在Ruby上加入其他表格的结果

Question

我想查询private_classes的结果。场景是：

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具有属性private_school_id。

我在做：

s = Student.find(9)
s.joined_private_schools

导致：

=> #<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">]>

如果我这样做：

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

它导致：

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">]> 

但这仍然是错误的结果。

我需要使用一个查询获得private_classes的结果，以便避免每个循环多个循环。

Answer 1

如果要拥有PrivateClass，则从错误的点开始（User）。您需要从PrivateClass开始加入所有中介表，然后使用用户ID作为条件：

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

您可以在关联上使用:through（至少在Rails 4.2中）
我更新了此，我首先错过了您想要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