title: How to use Rails 4 strong parameters with has_many :through association?
body: <p>I'm having trouble getting a has_many :through association working with Rails 4's strong parameters. I have a model called <code>Checkout</code> and I need to select a person from the <code>Employee</code> model in the new checkout form. Checkouts and Employees are associated through an <code>Employment</code> model.</p>
  
  <p>I'm getting this error when I try to create a new checkout:</p>
  
  <pre><code>NoMethodError in CheckoutsController#create
  undefined method `employee' for #&lt;Checkout:0x007ff4f8d07f88&gt;
  </code></pre>
  
  <p>It seems that there's something wrong with either my create action, my checkout parameters or my new checkout form. Here's the create action:</p>
  
  <pre><code>  def create    
      @user = current_user
      @checkout = @user.checkouts.build(checkout_params)
  
      respond_to do |format|
        if @checkout.save
          format.html { redirect_to @checkout, notice: 'Checkout was successfully created.' }
        else
          format.html { render action: 'new' }
        end
      end
    end
  </code></pre>
  
  <p>My checkout params:</p>
  
  <pre><code>def checkout_params
        params.require(:checkout).permit(:job, :employee_ids, :shift, :date, :hours, :sales, :tips, :owed, :collected, :notes)
  end
  </code></pre>
  
  <p>My new checkout form:</p>
  
  <pre><code>&lt;div class="field"&gt;
       &lt;%= f.label :employee %&gt;&lt;br&gt;
       &lt;%= f.collection_select(:employee_ids, Employee.all.collect, :id, :full_name, {:prompt =&gt; "Please select"} ) %&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>But I can't figure out what has changed with Rails 4 and strong parameters. In Rails 3 this type of association and form worked for me using attr_accessible instead of strong_parameters.</p>
  
  <p><strong>Relevant Files</strong></p>
  
  <p>Full Trace of the error:
  <a href="https://gist.github.com/leemcalilly/0cb9e2b539f9e1925a3d" rel="noreferrer">https://gist.github.com/leemcalilly/0cb9e2b539f9e1925a3d</a></p>
  
  <p>models/checkout.rb:
  <a href="https://gist.github.com/leemcalilly/012d6eae6b207beb147a" rel="noreferrer">https://gist.github.com/leemcalilly/012d6eae6b207beb147a</a></p>
  
  <p>controllers/checkouts_controller.rb:
  <a href="https://gist.github.com/leemcalilly/a47466504b7783b31773" rel="noreferrer">https://gist.github.com/leemcalilly/a47466504b7783b31773</a></p>
  
  <p>views/checkouts/_form.html.erb
  <a href="https://gist.github.com/leemcalilly/ce0b4049b23e3d431f55" rel="noreferrer">https://gist.github.com/leemcalilly/ce0b4049b23e3d431f55</a></p>
  
  <p>models/employee.rb:
  <a href="https://gist.github.com/leemcalilly/46150bee3e6216fa29d1" rel="noreferrer">https://gist.github.com/leemcalilly/46150bee3e6216fa29d1</a></p>
  
  <p>controllers/employees_controller.rb:
  <a href="https://gist.github.com/leemcalilly/04f3acdac0c9a678bca8" rel="noreferrer">https://gist.github.com/leemcalilly/04f3acdac0c9a678bca8</a></p>
  
  <p>models/employment.rb:
  <a href="https://gist.github.com/leemcalilly/6adad966dd48cb9d1b39" rel="noreferrer">https://gist.github.com/leemcalilly/6adad966dd48cb9d1b39</a></p>
  
  <p>db/schema.rb:
  <a href="https://gist.github.com/leemcalilly/36be318c677bad75b211" rel="noreferrer">https://gist.github.com/leemcalilly/36be318c677bad75b211</a></p>