title: Asynchronous updating list of options for multi select MVC
body: <p>I'm working on a form for user input, and one of the items (a multiple option select) has an inordinate amount of choices (~1600), so it's gotta get filtered down to be digestible. I've got 3 filter fields (dropdowns) that I'm requiring to have completed before I make an AJAX call back to the DB and get an updated list. It's similar to <a href="https://stackoverflow.com/questions/10304489/how-to-filter-the-options-of-a-drop-down-list-using-another-drop-down-list">How to filter the options of a drop down list using another drop down list</a>, however I also don't want to lose any items that were previously selected. Here's the signature for the function I've prototyped:</p>
  
  <pre><code>public JsonResult GetContentStandardsForUser(string type, string grade, string subject, List&lt;SelectListItem&gt; selected)
  </code></pre>
  
  <p>What I want is to return the new list of items (and not lose the ones that were already selected), and have the pick-list update.</p>
  
  <p>What is this AJAX call going to look like (using jquery)? Should I just include the current selected values in my query, or can I pass the <code>SelectListItem</code>s like I've written above?</p>