title: Django Admin, how to populate select options depending on another select
body: <p>My scenario:</p>
  
  <p>I'm having three tables, Category, Subcategory, Products.
  While inserting new product, there are two select boxes</p>
  
  <p>1)1st select is for Category (its working)</p>
  
  <p>2) 2nd is for Subcategory, which should be relevant to the 1st select. Needs to fetch the data from subcategory table.</p>
  
  <p>Subcategory table has category id as a foreign key.
  I am a beginner, please somebody help.</p>