具有多个联接的ActiveRecord查询无法识别关系

问题描述

我正在尝试编写一个ActiveRecord查询，该查询返回使用以下查询在某个课程中注册的所有学生：

  def self.students_enrolled_in（course_id）
学生
 .joins（：报名）
 .joins（：sections）
 .joins（：courses）
 .where（sections： {course_id：course_id}）
 end 
  

在rails控制台中的结果是：

似乎建立了关联。我究竟做错了什么？该查询实际上是不是意味着所有 join（）语句都必须与Student关联起来，还是应该ac找出关系链接？

教授显示页面：

 < div class = col -md-8> 
< h2 class = card-title><％= @ professor.name％>< / h2> 
 
<％@ courses_taught.each做|当然| ％> 
< div class = card mb-4 card-header> 
< img class = card-img-top src = http://placehold.it/750x300 alt =卡片图片上限> 
< h3 class = card-text><％= course.title％>< / h3> 
< / div> 
< div class = card-body> 
<％course.sections.enrollments.students.each do | student | ％> 
< p><％student.name％>< / p> 
<％end％> 
< / div> 
<％end％> 
 
< / div> 
  

型号：

注册

 班级注册< $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ ：
 班学生< ApplicationRecord 
 has_many：注册
结尾
  
教授：
  class Section< ApplicationRecord 
 has_many：注册
所属名称：教授
所属名称：课程
 
 validates_uniqueness_of：教授_id，范围：：course_id 
 
范围：by_professor_id， ->（prof_id）{where（'professor_id =？'，prof_id）} 
结尾
  
课程：
 班级课程< ApplicationRecord 
枚举状态：{规划：0，提供：1} 
 
范围：提供，-> {where（status：1）} 
范围：planning，-> {where（状态：0）} 
 
属于：部门
 has_many：sections 
 has_many：教授，通过：：sections 
 
验证：title， ：number，：status，：description，在线状态：true 
验证：description，长度：{in：10..500} 
验证：title，：number，uniqueness：{case_sensitive：false} 
 
 def self.search（term）
 if term 
 where（'title LIKE？'，％＃{term}％）。order（'title DESC'）
 else 
订单（'title ASC'）
结束
结束
 
 def self.taught_by（professor_id）
课程
 .joins （：sections）
 .joins（：professors）
 .where（sections：{Professor_id：Professor_id}）
 .select（'distinct course。*'）
 end 
 
结束
  
模式：
  ActiveRecord :: Schema.define（version：20171013201907）do 
 
 create_table courses，强制：：cascade do | t | 
 t.string title 
 t.text description 
 t.string number 
 t.datetime created_at，null：false 
 t。 datetime updated_at，空：false 
 t.integer status，默认值：0 
 t.integer department_id 
 t.index [ department_id]，名称： index_courses_on_department_id 
 end 
 
 create_table部门，强制：：cascade do | t | 
 t.string名称 
 t.text描述 
 t.text main_image 
 t.text thumb_image 
 t.datetime created_at ，空值：false 
 t.datetime updated_at，空值：false 
 end 
 
 create_table enrollments，强制：：cascade do | t | 
 t.integer section_id 
 t.integer student_id 
 t.datetime created_at，null：false 
 t.datetime updated_at，null：false 
 t.index [ section_id]，名称： index_enrollments_on_section_id 
 t.index [ student_id]，名称： index_enrollments_on_student_id 
 end 
 
 create_table教授，强制：：cascade do | t | 
 t.string名称 
 t.datetime created_at，null：false 
 t.datetime updated_at，null：false 
 t.integer status，默认：0 
 t.integer department_id 
 t.text bio 
 t.index [ department_id]，名称： index_professors_on_department_id 
 end 
 
 create_table sections，强制：：cascade do | t | 
 t.integer number 
 t.integer max_enrollment 
 t.datetime created_at，null：false 
 t.datetime updated_at，null：false 
 t.integer教授_id 
 t.integer课程ID 
 t.string房间 
 t.index [ course_id]，名称： index_sections_on_course_id 
 t.index [ professor_id， course_id]，名称： index_sections_on_professor_id_and_course_id，唯一：true 
 t.index [ professor_id]，名称： index_sections_on_professor_id 
 end 
 
 create_table students，强制：：cascade do | t | 
 t.string名称 
 t。十进制 gpa 
 t.datetime created_at，null：false 
 t.datetime updated_at，null：false 
 end 
 
 create_table users，强制执行：：cascade do | t | 
 t.string email，默认值：，null：false 
 t.string encrypted_pa​​ssword，默认值：，null：false 
 t.string name 
 t.string reset_password_token 
 t.datetime reset_password_sent_at 
 t.datetime remember_created_at 
 t.integer sign_in_count，默认值：0，null：false 
 t.datetime current_sign_in_at 
 t.datetime last_sign_in_at 
 t.string current_sign_in_ip 
 t.string last_sign_in_ip 
 t.datetime created_at，null：false 
 t.datetime updated_at，空：false 
 t.string角色 
 t.index [ email]，名称： index_users_on_email，唯一：true 
 t.index [ reset_password_token]，名称： index_users_on_reset_password_token，唯一：true 
结束
 
结束
  
 
 
解决方案
您正在过度申请 .joins 。尝试从内而外开始。首先，找到课程：
  Course.find_by（id：course_id）
  
然后，找到与课程相关的所有部分。无需在这里执行 join  
  Section.where（course ：Course.find_by（id：course_id））
  
现在，您要加入：
  Student.joins（：enrollments）.where（注册人数：{section：Section.where（course：Course.find_by（id：course_id））}} ）
  
我认为应该可以帮到您。但是，未经测试。因此，放手一试，看看它是否有效。
 
 
 
 P.S .：尝试仅发布最相关的代码。整理一大堆无关紧要的东西并不是那么有趣。 
 
I am trying to write an ActiveRecord Query that returns all students enrolled in a certain course with the following query:
def self.students_enrolled_in(course_id)
    Student
        .joins(:enrollments)
        .joins(:sections)
        .joins(:courses)
        .where(sections: { course_id: course_id })
  end
the result in the rails console is:
it seems that the association is made. what am I doing wrong? does the query actually mean that all the join() statements have to relate back to Student, or should ac trace out the relational links? 
Professor show page:
<div class="col-md-8">
  <h2 class="card-title"><%= @professor.name %></h2>

    <% @courses_taught.each do |course| %>
        <div class="card mb-4 card-header">
          <img class="card-img-top" src="http://placehold.it/750x300" alt="Card image cap">
          <h3 class="card-text"><%= course.title %></h3>
        </div>
        <div class="card-body">
          <% course.sections.enrollments.students.each do |student| %>
              <p><% student.name %></p>
          <% end %>
        </div>
    <% end %>

</div>
models:
enrollment
class Enrollment < ApplicationRecord
  belongs_to :section
  belongs_to :student
end
Student:
class Student < ApplicationRecord
  has_many :enrollments
end
Professor:
class Section < ApplicationRecord
  has_many :enrollments
  belongs_to :professor
  belongs_to :course

  validates_uniqueness_of :professor_id, scope: :course_id

  scope :by_professor_id, ->(prof_id) { where('professor_id = ?', prof_id) }
end
Course:
class Course < ApplicationRecord
  enum status: { planning: 0, offered: 1 }

  scope :offered, -> { where(status: 1) }
  scope :planning, -> { where(status: 0) }

  belongs_to :department
  has_many :sections
  has_many :professors, through: :sections

  validates :title, :number, :status, :description, presence: true
  validates :description, length: { in: 10..500 }
  validates :title, :number, uniqueness: { case_sensitive: false }

  def self.search(term)
    if term
      where('title LIKE ?', "%#{term}%").order('title DESC')
    else
      order('title ASC')
    end
  end

  def self.taught_by(professor_id)
    Course
        .joins(:sections)
        .joins(:professors)
        .where(sections: { professor_id: professor_id })
        .select('distinct courses.*')
  end

end
Schema:
ActiveRecord::Schema.define(version: 20171013201907) do

  create_table "courses", force: :cascade do |t|
    t.string "title"
    t.text "description"
    t.string "number"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "status", default: 0
    t.integer "department_id"
    t.index ["department_id"], name: "index_courses_on_department_id"
  end

  create_table "departments", force: :cascade do |t|
    t.string "name"
    t.text "description"
    t.text "main_image"
    t.text "thumb_image"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "enrollments", force: :cascade do |t|
    t.integer "section_id"
    t.integer "student_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["section_id"], name: "index_enrollments_on_section_id"
    t.index ["student_id"], name: "index_enrollments_on_student_id"
  end

  create_table "professors", force: :cascade do |t|
    t.string "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "status", default: 0
    t.integer "department_id"
    t.text "bio"
    t.index ["department_id"], name: "index_professors_on_department_id"
  end

  create_table "sections", force: :cascade do |t|
    t.integer "number"
    t.integer "max_enrollment"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "professor_id"
    t.integer "course_id"
    t.string "room"
    t.index ["course_id"], name: "index_sections_on_course_id"
    t.index ["professor_id", "course_id"], name: "index_sections_on_professor_id_and_course_id", unique: true
    t.index ["professor_id"], name: "index_sections_on_professor_id"
  end

  create_table "students", force: :cascade do |t|
    t.string "name"
    t.decimal "gpa"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

  create_table "users", force: :cascade do |t|
    t.string "email", default: "", null: false
    t.string "encrypted_password", default: "", null: false
    t.string "name"
    t.string "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer "sign_in_count", default: 0, null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string "current_sign_in_ip"
    t.string "last_sign_in_ip"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.string "roles"
    t.index ["email"], name: "index_users_on_email", unique: true
    t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
  end

end
 解决方案 
You're over-applying .joins. Try starting from the inside out. First, find the course:
Course.find_by(id: course_id)
Then, find all the sections associated with the course. No need to do a joins here:
Section.where(course: Course.find_by(id: course_id))
Now you do your join:
Student.joins(:enrollments).where(enrollments: {section: Section.where(course: Course.find_by(id: course_id))})
I think that ought to do the trick for you. But, untested. So, give it a go and see if it works.
P.S.: Try posting only the most relevant code. It's not so much fun to sort through a bunch of extraneous stuff. 
                        
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