为关联表添加 JOIN [重复]

Question

我的 Organization 类如下所示：

has_many Students

我的学生班级如下所示：

has_many Klasses
belongs_to Organization

我的 Klass 类如下所示：

some field named : price
scope :top_expensive_classes, joins(:students).order('price DESC')
belongs_to Student

我的查询如下所示：

@results = Klass.top_expensive_classes.where(organization_id: params[:id]).limit(RESULT_SET_COUNT)

注意它以 Klass 开头，所以这就是问题所在，因为我在 where 类中搜索 organization_id 但那不在 Klass 中，它在 Student 类中，所以我应该以某种方式在某处引入一个 join 来解决这个问题，但是想不通。

Answer 1

我认为真正的问题是您的关联可能不正确。

• 一个学生有很多课
• 一个班级有很多学生

但你拥有的是

• 一个学生有很多课
• 一个班级属于一个学生

这真的没有意义（至少在我见过的任何情况下班级和学生互动）。您应该在 Klass 和 Student 之间创建多对多关系，而不是一对多关系。

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

一旦你有了这些正确的关联，你需要在Klass类的:students关联上调用.joins。没有范围也可以。

Klass.joins(:students).where("students.organization_id = ?", params[:id]).order('price DESC').limit(RESULT_SET_COUNT)

在 ActiveRecord 查询中阅读 the guide。

---

这是证明（使用上面的确切模型定义），关联的顺序确实不重要。

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]