title: Rails simple_form association not saving to db
body: <p>I have 2 models with a HABTM relationship: authors and publications.</p>
  
  <p>Using simple_form, I was able to create a <code>add new publication</code> form that shows all of the possible authors as checkboxes. However, these associations are not saving to my db.</p>
  
  <p>I know this is a params permit issue, but I'm not sure how to properly write it. Currently I have:</p>
  
  <pre><code>def publication_params
      params.require(:publication).permit(:publication_id, :name, :year_published, :month_published, :day_published, :citation, :source, :abstract)
  end
  </code></pre>
  
  <p>How do I write this so that rails can allow the association from the authors table (which has only two columns-- <code>author_id</code> and <code>name</code>) to be saved too?</p>