many-to-many with has_many :through association nested form - ruby-on-rails

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
enrollment/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 %>
By referering to :
many-to-many: has_many :through association form with data assigned to linking model create form view
Intention: Create the Enrollments with the existing courses and students.
Current implementation of enrollment/new.html.erb will show all of the existing enrollments on the form which is not the desired presentation view.
I wish to create a blank form for creating enrollments.
How can I do that?

Answer 1

By just adding a line, "!e.object.persisted?" it solves the problem.
enrollment/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 %>