title: attr_accessor usage for a custom attribute of ActiveModel
body: <p>I need to implement a form with a <strong>custom field</strong> (which doesn't have it's reference in db) using a checkbox (boolean field).</p>
  
  <p>if the checkbox is checked, the backend must write a certain many-to-many row and point it to the updated object.</p>
  
  <p>The problem is my setter method in <strong>article.rb</strong> is not being accessed on form post.</p>
  
  <p><strong>article.rb</strong></p>
  
  <pre><code>class Article &lt; ActiveRecord::Base
    attr_accessor :highlight
  
    def highlight
    end
  
    def highlight=(arg)
      puts "NO ACCESS"
    end
  end
  </code></pre>
  
  <p><strong>_form.html.erb</strong></p>
  
  <pre><code>&lt;%= semantic_form_for [:admin, @article] do |f| %&gt;
    &lt;%= f.input :title, :label =&gt; "Title" %&gt;
    &lt;%= f.input :text, :as =&gt; :ckeditor %&gt;
    &lt;%= f.input :highlight, :as =&gt; :boolean, :label =&gt; "Highlight" %&gt;
  </code></pre>
  
  <p><strong>article.rb(active_admin)</strong></p>
  
  <pre><code>ActiveAdmin.register Article do
    form partial: 'form'
    permit_params do
      permitted = [:title, :text, :highlight]
      permitted
    end
  end
  </code></pre>