多到多用的has_many：通過關聯嵌套形式

Question

student.rb

class Student < ActiveRecord::Base 
    has_many :enrollments 
    has_many :courses, through: :enrollments 

    accepts_nested_attributes_for :enrollments 
end 

enrollment.rb

class Enrollment < ActiveRecord::Base 
    belongs_to :student 
    belongs_to :course 
end 

course.rb

class Course < ActiveRecord::Base 
    has_many :enrollments 
    has_many :students, through: :enrollments 
end 

enrollments_controller.rb

class EnrollmentsController < ApplicationController 

    def new 
    @current_student = current_user.student 
    @enrollments = @current_student.enrollments.build 
    end 

    def create 
    current_student = current_user.student 
    @enrollments = current_student.enrollments.build(enrollment_params) 
     if @enrollments.save 
      flash[:success] = "You have successfully enrolled." 
      redirect_to new_enrollment_path 
     else 
      # fail 
      flash[:danger] = "Please try again." 
      redirect_to new_enrollment_path 
     end 
    end 

    private 
    def enrollment_params 
     params.require(:enrollment).permit(:student_id, :course_id) 
    end 

end 

註冊/ new.html.erb

<%= nested_form_for(@current_student, html: { class: 'form-horizontal' }) do |f| %> 

    <%= f.fields_for :enrollments do |e| %> 
    <div class="form-group"> 
     <%= e.label :course_id, for: :course_id, class: 'col-xs-3 control-label' %> 
     <div class="col-xs-9"> 
      <%= e.collection_select :course_id, Course.all, :id, :name, {prompt: "Select your Course"}, {class: 'form-control'} %> 
     </div> 
    </div> 
    <% end %> 

    <%= f.link_to_add 'Add Course', :enrollments, class: "col-xs-9 col-xs-offset-3" %> 

    <div class="form-group"> 
     <div class="col-xs-9 col-xs-offset-3"> 
     <%= f.submit "Enroll Now", class: 'btn btn-primary' %> 
     </div> 
    </div> 

<% end %> 

通過referering到：

many-to-many: has_many :through association form with data assigned to linking model create form view

意向：與現有課程和學生創建擴招。

當前的enrollment/new.html.erb實現將顯示錶單上不是所需演示視圖的所有現有註冊。

我希望創建一個空白表單來創建註冊。 我該怎麼做？

Answer 1

只需添加一行，「！e.object.persisted？」它解決了這個問題。

註冊/ new.html.erb

<%= f.fields_for :enrollments do |e| %> 

    <!-- --> 
    <% if !e.object.persisted? %> 
     <div class="form-group"> 
     <%= e.label :course_id, for: :course_id, class: 'col-xs-3 control-label' %> 
      <div class="col-xs-9"> 
      <%= e.collection_select :course_id, Course.all, :id, :name, {prompt: "Select your Course"}, {class: 'form-control'} %> 
      </div> 
     </div> 
    <% end %> 
<% end %>