Getting a Controller, Route and Hidden-Field-Button to work correctly in Rails
===

I would like to create a button so that a user can enroll in a course. The enrolling model belongs_to users and courses and should keep track of enrollment. 

I have the following code displayed as a button-form on a course page (looks fine):

    <% @enroll = current_user.enrollings.build(course_id: @course.id) %>
	<%= form_for(@enroll) do |f| %>  
  		<div><%= f.hidden_field :course_id %></div>
  		<%= f.submit "Enroll", class: "btn btn-large btn-primary" %>
	<% end %>

I have a resources route for enrollings, and when the user clicks on the button, the page goes to /enrollings. 

This is the code in the enrolling controller:

    def create    
    @course = Course.find(params[:course_id])
    current_user.enroll!(@course)
    end

When I click on the button, I get the following error message:

> ActiveRecord::RecordNotFound in EnrollingsController#create
>
> Couldn't find Course without an ID

Why isn't it passing the course_id with the form? How do I setup the controller correctly? 

