title: Rails_admin, change values of one field by value of another
body: <p>I have three models: <code>Category</code>, <code>SubCategory</code> and <code>Item</code>.</p>
  
  <p><code>SubCategory</code> belongs to <code>Category</code></p>
  
  <p><code>Item</code> belongs to <code>Category</code> and <code>SubCategory</code></p>
  
  <p>In rails_admin's <code>create</code> and <code>edit</code> page, where I can assign values for items, values of categories and subcategories are shown as <strong>enums of ALL existing Categories and SubCategories</strong>.</p>
  
  <p>When I create new <code>Item</code>, I will select the <code>Category</code>. I want to keep only values of <code>SubCategories</code>, which is belongs to selected category, and only be able to select them.</p>
  
  <p>How it is possible to make rails_admin to select SubCategories values depending on what Category is selected?</p>