问题
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:
ActiveRecord::ConfigurationError: Can't join 'Student' to association named 'sections'; perhaps you misspelled it?
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
回答1:
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.
回答2:
Another way to do this is to add some more associations to your Student model:
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
You can then find all students enrolled in a course with:
Student.enrolled_in_course(course)
来源：https://stackoverflow.com/questions/46859024/activerecord-query-with-multiple-joins-not-recognizing-the-relations