如何在has_many中通过关联执行创建和更新？

Question

Note :

I put all of my source code for this app here .

---

I have this inside my schema.rb file:

create_table "courses", force: :cascade do |t|
    t.integer  "teacher_id"
    t.integer  "student_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer  "quantity"
  end

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

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

My teacher model:

class Teacher < ActiveRecord::Base
    has_many :courses
    has_many :students, :through => :courses
end

My student model:

class Student < ActiveRecord::Base
    has_many :courses
    has_many :teachers, :through => :courses
end

My course model:

class Course < ActiveRecord::Base
    belongs_to :teacher
    belongs_to :student
end

And I have empty add_student method inside my teachers_controller like this:

def add_student

end

The add_student.html.erb inside views/teachers folder:

hello from add_student!

<%= form_for(@course) do |f| %>
  <% if @course.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@course.errors.count, "error") %> prohibited this course from being saved:</h2>

      <ul>
      <% @course.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.select :student_id, Student.all.collect { |p| [ p.name, p.id ] } %>
  </div>
  <div class="field">
      <%= f.number_field :quantity %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

What I'm trying to do inside this add_student is adding students and quantity accordingly.

But I get this error:

First argument in form cannot contain nil or be empty

Which refers to this line:

<%= form_for(@course) do |f| %>

My questions :

1. How to display all students name in a dropdown and give them quantity inside this add_student method?

2. How to make this also works using POST request (like an API) so that I can simply POST params like this:

[{"student_id":1, "quantity":2},{"student_id":2, "quantity":3}]

3. How to make the update method for this add_student using form and PUT request so that I can pass the same style of params like such?

[{"student_id":1, "quantity":2},{"student_id":2, "quantity":3}]

Note #1 :

This is all my routes:

Prefix Verb   URI Pattern                         Controller#Action
        root GET    /                                   courses#index
     courses GET    /courses(.:format)                  courses#index
             POST   /courses(.:format)                  courses#create
  new_course GET    /courses/new(.:format)              courses#new
 edit_course GET    /courses/:id/edit(.:format)         courses#edit
      course GET    /courses/:id(.:format)              courses#show
             PATCH  /courses/:id(.:format)              courses#update
             PUT    /courses/:id(.:format)              courses#update
             DELETE /courses/:id(.:format)              courses#destroy
    students GET    /students(.:format)                 students#index
             POST   /students(.:format)                 students#create
 new_student GET    /students/new(.:format)             students#new
edit_student GET    /students/:id/edit(.:format)        students#edit
     student GET    /students/:id(.:format)             students#show
             PATCH  /students/:id(.:format)             students#update
             PUT    /students/:id(.:format)             students#update
             DELETE /students/:id(.:format)             students#destroy
    teachers GET    /teachers(.:format)                 teachers#index
             POST   /teachers(.:format)                 teachers#create
 new_teacher GET    /teachers/new(.:format)             teachers#new
edit_teacher GET    /teachers/:id/edit(.:format)        teachers#edit
     teacher GET    /teachers/:id(.:format)             teachers#show
             PATCH  /teachers/:id(.:format)             teachers#update
             PUT    /teachers/:id(.:format)             teachers#update
             DELETE /teachers/:id(.:format)             teachers#destroy
             GET    /teachers/:id/add_student(.:format) teachers#add_student

Note #2

I put all of my source code for this app here .

Answer 1

pleas do this in your teachers_controller

 def add_student
    @course = Course.new
 end

currently your method is empty and @course is nil

Answer 2

To make post or update request you need to pass url and method options to form_for tag. Follow followings step:-

1. Inside add_student method, initialize @course variable.

2. overwrite the from tag with options value as:-

   To create the record: <%= form_for(@course, url: courses_path, method: :post) do |f| %>

   To update the record: <%= form_for(@course, url: course_path(@course), method: :put) do |f| %>

Thanks