Rails: undefined method `cover_letter' error

In my rails application I have this three models User, Job and Job_application the association between the 3 is below

class Job < ActiveRecord::Base

  has_many :job_applications
  has_many :applicants, :through => :job_applications, :source => :user   

  accepts_nested_attributes_for :job_applications

class JobApplication < ActiveRecord::Base
  belongs_to :user
  belongs_to :job

class User < ActiveRecord::Base
  has_many :job_applications  
  has_many :applied_jobs, :through => :job_applications, :source => :job

In my Job controller I have this method so that user can apply to job

  def apply
    @job = Job.find_by_slug(params[:id])  
    @job_application = JobApplication.new
    @job_application.user_id = current_user.id
    @job_application.job_id = @job.id

    if @job_application.save
      JobMailer.somebody_applied(@job).deliver
      redirect_to job_url(@job)
    else
      render :action => "show"
    end
  end

and my Job view page I have this code for the apply method

<button type="button" id="control_gen_5" data-toggle="modal" data-target="#myModal">
  <span>Envoyer un candidature</span>
</button>

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close close-this" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Apply to <%= @job.job_title %></h4>
      </div>
      <p><%= current_user.full_name%></p>
      <%= form_for @job, :url => apply_job_path(@job), :method => :post, :html => { :multipart => true } do |f| %>
        <%= f.fields_for :job_applications, @job.job_applications.build do |s| %>
          <%= s.hidden_field :user_id %>
          <%= s.hidden_field :job_id %>
          <div class="modal-body">
            <p>
              <div class="form-inputs">
                <%= s.text_area :cover_letter, :input_html => { :rows => 5, :class => 'span12', :value => cookies[:cover_letter] }, :label => false %>
              </div>
            </p>
          </div>
        <% end %>
        <div class="modal-footer">
          <%= f.submit 'Postuler', :class => 'btn btn-info', :name => nil %>
        </div>
      <% end %>
    </div>
  </div>
</div>

I have also this code so I can view who has applied for a job and their cover letter

</ul>
<% @job.job_applications.each do |job_application| %>
  <li>
    <%= job_application.user.full_name %>
    <%= job_application.cover_letter %>
  </li>
<% end %>
</ul>

But the is that only the user full name are displayed and not the cover letter.

I have tried this code in my rails console

j.job_applications.last   where   j = Job.last

this has give me this

#<JobApplication id: 42, job_id: 24, user_id: 49, cover_letter: nil,  created_at: "2015-04-20 14:22:31", updated_at: "2015-04-20 14:22:31">

Any one can help to know why I'm getting nil for cover_letter