具有多个连接的 ActiveRecord 查询无法识别关系

Question

我正在尝试编写一个 ActiveRecord 查询，它使用以下查询返回所有参加某门课程的学生：

def self.students_enrolled_in(course_id)
    Student
        .joins(:enrollments)
        .joins(:sections)
        .joins(:courses)
        .where(sections: { course_id: course_id })
  end

rails 控制台中的结果是：

ActiveRecord::ConfigurationError: 无法将 'Student' 加入名为 'sections' 的关联；也许你拼错了？

似乎已经建立了关联。我究竟做错了什么？查询实际上是否意味着所有join() 语句都必须与学生相关，或者应该追踪关系链接？

教授展示页面：

<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>

型号：

注册

class Enrollment < ApplicationRecord
  belongs_to :section
  belongs_to :student
end

学生：

class Student < ApplicationRecord
  has_many :enrollments
end

教授：

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

课程：

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

架构：

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

Answer 1

另一种方法是为您的Student 模型添加更多关联：

class Student < ApplicationRecord
  has_many :enrollments
  has_many :sections, through: :enrollments
  has_many :courses, through: :sections

  scope :enrolled_in_course, -> (course) { joins(:sections).where(course_id: course.id)
end

然后您可以通过以下方式找到所有注册course 的学生：

Student.enrolled_in_course(course)

Answer 2

您过度申请.joins。尝试从内到外。首先，找到课程：

Course.find_by(id: course_id)

然后，找到与course 关联的所有部分。无需在这里做joins：

Section.where(course: Course.find_by(id: course_id))

现在你加入：

Student.joins(:enrollments).where(enrollments: {section: Section.where(course: Course.find_by(id: course_id))})

我认为应该对你有用。但是，未经测试。所以，试一试，看看它是否有效。

P.S.：尝试只发布最相关的代码。整理一堆无关紧要的东西并不是那么有趣。