 <p>You don't want to use <code>@course.chapters.build</code> here because this does add an empty chapter to the course. Instead you'll want to use <code>Chapter.new</code> and set the <code>:course</code> option like this.</p> <pre><code>@newchapter = Chapter.new(:course =&gt; @course) </code></pre> <p>It may not even be necessary to specify <code>:course</code> here depending on how you are using <code>@newchapter</code>.</p>