title: How do I check boxes in a collection for existing records in simple form?
body: <p>Say I have an input field like this:</p>
  
  <pre><code>&lt;%= f.input :parents, collection: @nodes, as: :check_boxes, label: "Parents" %&gt;
  </code></pre>
  
  <p>What I would like to happen is for any <code>@nodes</code> that are already <code>node.parents</code> of the existing node, rather than showing their checkbox as empty, I would like it to have a checkbox in it.</p>
  
  <p>The idea being that I should be able to uncheck them and check others to change the parents of the existing node.</p>