title: Sitecore query filter issue
body: <p>I am new to Sitecore and have come across a situation for which I haven't been able to find any documentation. </p>
  
  <p>I have two blogs, <code>Blog A</code> and <code>Blog B</code>. Each Blog has various <code>Categories</code>. </p>
  
  <p>Now, I am trying to create a template, where the user can select a Blog and then select Categories. For Bog Selection, I have used a <strong><em>Droptree</em></strong> and for Categories Selection, I have used a <strong><em>Multilist</em></strong>. I can fetch all the Categories using the following query :</p>
  
  <pre><code>query:../../..//*[@@templatename= 'Category']
  </code></pre>
  
  <p>But, this fetches all the categories belonging to both the Blogs. What I want to do is - when the user has selected the Blog in the DropTree, I want to populate the Multilist only with the categories belonging to that particular blog. </p>
  
  <p>Any ideas? Thanks!</p>