如何关联已连接表中的记录？ -Ruby on Rails

Question

So Basically I Have 3 Tables that are associated together and one extra table for the table with each other: Students, Teachers, Subjects and Enrolled Subjects:

Here are my migrations:

class CreateStudents < ActiveRecord::Migration[5.0]

  def up
  create_table :students, :id => false do |t|
    t.integer "student_id", :auto_increment => true, :primary_key => true
    t.string "first_name", :limit => 25
    t.string "last_name", :limit => 50
    t.string "email", :default => ' ', :null => false
    t.string "birthday"
    t.string "username", :limit => 25
    t.string "password_digest"
    t.timestamps
  end
  end

class CreateTeachers < ActiveRecord::Migration[5.0]
  def up
  create_table :teachers, :id => false do |t|
      t.integer "teacher_id", :auto_increment => true, :primary_key => true
      t.string "first_name"
      t.string "last_name"
      t.string "email", :default => ' ', :null => false
      t.string "birthday"
      t.string "username", :limit => 25
      t.string "password_digest"
      t.timestamps
    end
  end

class CreateSubjects < ActiveRecord::Migration[5.0]
  def up
    create_table :subjects, :id => false do |t|
       t.integer "subject_id", :auto_increment => true, :primary_key => true
       t.string "subject_name"
       t.timestamps
    end
  end

class CreateEnrolledSubjects < ActiveRecord::Migration[5.0]
  def up
    create_table :enrolled_subjects, :id => false do |t|
      t.integer "subject_id"
      t.integer "teacher_id"
      t.integer "student_id"
    end
  end

So basically:

• One students can have many subjects
• One teacher can have many subjects
• One students can have many teachers

So I formulated this on my models:

class Student < ApplicationRecord

  has_many :enrolled_subjects
  has_many :subjects, through: :enrolled_subjects
  has_many :teachers, through: :enrolled_subjects

  has_many :admin_users
  has_secure_password

end

class Teacher < ApplicationRecord

  has_many :enrolled_subjects

  has_many :subjects, through: :enrolled_subjects
  has_many :students, through: :enrolled_subjects
  has_many :admin_users
  has_secure_password

end

class Subject < ApplicationRecord

  has_many :students, through: :enrolled_subjects
  has_many :teachers, through: :enrolled_subjects
  has_many :admin_users

end

class EnrolledSubject < ApplicationRecord
  belongs_to :student
  belongs_to :subject
  belongs_to :teacher
end

Now to put the association on my students, teachers, subjects with my existing records I pull up my records for both on Rails Console:

• students -> subjects
• teachers -> subjects
• students -> teachers

USING RAILS CONSOLE:

 stud = Student.find(1)
    subj = Subject.find(1)
    stud = Student.Subject << subj

    teacher = Teacher.find(1)
    subj = Subject.find(1)
    teacher = Teacher.Subject << subj

BUT I GOT THE FOLLOWING ERRORS:

 NoMethodError: undefined method `Subject' for #<Class:0x007fcf47229980>
        from /Users/mac/.rvm/gems/ruby-2.3.1/gems/activerecord-5.0.1/lib/active_record/dynamic_matchers.rb:21:in `method_missing'

Next, I also tried to pull up the student's subjects and teachers (on my views) using the ff:

<% @students.each do |student|%>
<%= student.teachers %>
<%= student.subjects %>
<% end %>

<% @teachers.each do |teacher|%>
<%= teacher.subjects %>
<% end %>

BUT IT ONLY RETURNS ERROR:

ActiveRecord_Associations_CollectionProxy:0x007fed07a7a9a8

1. How can I associate the existing records on both students and tables (on Rails Console)
2. How can I display the records especially the Student's enrolled subjects and their teachers. And for teachers, how can I display their subjects. (via views)

Answer 1

First of all, I highly recommend you read this guide in its entirety. It's a thorough explanation of associations and help you understand what's going on.

---

But for your case, first the console. Look at what the error message is saying:

NoMethodError: undefined method `Subject' for #<Class:0x007fcf47229980>

This means there are two issues here.

First, you're finding the student and the subject properly, using their respective classes

 stud = Student.find(1)
 subj = Subject.find(1) 

But then when you try to add a subject to a student, you go back to the class. You need to use the objects you found, which are now stored in stud and subj .

Furthermore, because your association is has_many , the method name must be plural. So the third line should look like this:

stud.subjects << subj

---

Second, your display. It's not returning an error, it's returning a Ruby object, which is an active record collection. So you have what you need, but now you should go a step further.

The common case is to use the each method on the collection to iterate through it and display what you'd like. For example:

<% @students.each do |student|%>
  <% student.teachers.each do |teacher| %>
    <%= teacher.first_name %>
  <% end %>
<% end %>

But, beware of N+1 query problems . If you're going to iterate over objects like this, you should eager load the associated objects .

---

And one other thing, as I look through your code. Unless you have a specific and compelling reason to do so, don't change the id field. With Rails, it's best to follow the convention over configuration approach and let Rails work for you.

Answer 2

您应该在查询中使用stud.subjects ，因为学生有很多科目，因此必须使用复数形式。