An simple Gist showing how to add students to courses through an enrollments table with a collection select.

about.md

To generate this project, do the following on the command line:

rails generate scaffold student name:string

rails generate scaffold course name:string

rails generate model enrollment student_id:integer course_id:integer

rake db:migrate

Feel free to submit a pull request, or ask any questions!

course.rb

class Course < ActiveRecord::Base
  attr_accessible :name, :student_ids #student_ids is from the collection_select in the view file below
  
  has_many :enrollments
  has_many :students, through: :enrollments
  
  # A course may have many students through the join table called enrollments. 
end

courses_controller.rb

class CoursesController < ApplicationController

  # For simplicity, I left out all the other actions in the controller that are generated by rails because you don't need to edit them fro this.
  # I left out: index, show, new, edit, create, destroy
  
  # PUT /courses/1
  # PUT /courses/1.json
  def update
    @course = Course.find(params[:id])

### Added code ###
    student_id = params[:course].delete(:students)

    if student_id # This if statement makes sure a registration isn't created every time the a course is updated.
      registration = Registration.create(:student_id => student_id, :course_id => @course.id) #actual creation of the registration
    end
### End added code ###

    respond_to do |format|
      if @course.update_attributes(params[:course])
        format.html { redirect_to @course, notice: 'Course was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @course.errors, status: :unprocessable_entity }
      end
    end
  end
  
end

enrollment.rb

class Enrollment < ActiveRecord::Base
  attr_accessible :student_id, :course_id
  
  belongs_to :student
  belongs_to :course
end

show.html.erb

# In app/views/courses
<p>
  <b>Name:</b>
  <%= @course.name %>
</p>

<!-- The below code just shows which students are enrolled. Since you are pulling from a join table, you have to query the 
database to find the name of the student from his id -->
<p>
  <b>Enrolled Students</b><br>
		<% @course.enrollments.each do |enrollment| %>
  		<%= Student.find(enrollment.student_id).name %><br>
  	<% end %>
 </p>

<b> Add a student to this course </b>
<%= form_for(@course) do |f| %>
  <%= f.collection_select 'student_ids', Student.all, :id, :name, { include_blank: "" } %>
  <%= f.submit %>
<% end %>

<!-- 
collection_select creates a dropdown menu
student_ids is the field in the table that selection will be written to. Note that it's plural because you can add multiple courses
Student.all lists all the students from the students table in the drop down
:id is the actual value from the students table that will be recorded in the student_id field in the registrations table
:name is what users will see as their option to select in the dropdown
{ include_blank: "" } just puts a blank space as the default in the drop down
-->

show2.html.erb

<!-- In app/views/students -->
<!-- Note, the actual name of the file is show.html.erb. Gist won't let me have two files with the same name -->
<p>
  <b>Name:</b>
  <%= @student.name %>
</p>

<!-- The below code just shows which courses the student is enrolled in. Since you are pulling from a join table, you have to query the 
database to find the name of the course from its id -->
<p>
  <b>Courses enrolled in</b><br>
		<% @student.enrollments.each do |enrollment| %>
  		<%= Course.find(enrollment.course_id).name %><br>
  	<% end %>
 </p>

<b> Add this student to a course </b>
<%= form_for(@student) do |f| %>
	<%= f.collection_select 'course_ids', Course.all, :id, :name, { include_blank: "" } %>
	<%= f.submit %>
<% end %>

<!-- 
collection_select creates a dropdown menu
course_ids is the field in the table that selection will be written to. Note that it's plural because you can add multiple students
Course.all lists all the courses from the courses table in the drop down
:id is the actual value from the courses table that will be recorded in the course_id field in the registrations table
:name is what users will see as their option to select in the dropdown
{ include_blank: "" } just puts a blank space as the default in the drop down
-->

student.rb

class Student < ActiveRecord::Base
  attr_accessible :name, :course_ids #course_ids is from the_collection select in the view file below
  
  has_many :enrollments #Join table
  has_many :courses, through: :enrollments
  
  # A student can be enrolled in many classes through the join table called enrolments
end

students_controller.rb

class StudentsController < ApplicationController  
  
  # Again, for simplicity, I left out all the other actions in the controller that are generated by rails because you don't need to edit them fro this.
  # I left out: index, show, new, edit, create, destroy
  
  # PUT /students/1
  # PUT /students/1.json
  def update
    @student = Student.find(params[:id])
    
### Added code ###
    course_id = params[:student].delete(:courses)

    if course_id # This makes sure a registration isn't created every time the a student is updated.
      registration = Registration.create(:course_id => course_id, :student_id => @student.id)
    end
### End added code ###    

    respond_to do |format|
      if @student.update_attributes(params[:student])
        format.html { redirect_to @student, notice: 'Student was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @student.errors, status: :unprocessable_entity }
      end
    end
  end

end