MVC2 && EF4.0 Cascading Drop Down List

I am trying to create a cascading drop down in MVC2. This is a simple application so I am using Entity Framework 4.0 classes in my User Layer (instead of the usual POCOs).

I looked at Steve’s Walther’s solution(s) here: the problem is that it doesn’t work – you get an object null reference b/c the page isn’t done loading (I think). I tried putting it into JQuery, which I know waits to fire until the DOM is loaded, but I started getting too deep into the port.

I then looked at other people’s attempts like here and here – lots of code, little explanation and it doesn’t work without modifications.

I then went back to a project that I created does an Ajax call for the second drop down and tried to create my own solution.

Here is the initial controller:

1         public JsonResult GetDoctrinesForAnIssue(int issueId)
2         {
3             IDoctrineFactory doctrineFactory = new DoctrineFactory();
4             List<Doctrine> doctrines = doctrineFactory.GetDoctrines(issueId);
5             return Json(doctrines.ToList());
6         }
7  

The problem is that I am using an EF class directly in the JSON return. I am getting a missing object context because the entire graph is not loading and the context is disposed when it is going back to rehydrate – one of limitations of EF is that the instantiated class has to exist with the object context that created it so when that context goes away, you are stuck.

When I put the entire graph in to see, I got a circular reference error. Ugh

1                 var doctrineQuery = (from doctrine in patentEntities.Doctrines
2                                      .Include("Issue")
3                                      .Include("Comment")
4                                      .Include("Comment.Claim")
5                     .etc….
6                                      select doctrine).ToList<Doctrine>();
7                 return doctrineQuery;
8  

I then thought about some of my other projects that I have worked on lately. I used POCOs extensively so this kind of problem didn’t surface – the downside is that I would then need to code up the POCOs, the Factories, and the mapping between the POCOs and EF classes – which is a bit overkill for this project. I then thought of a third way of using a dynamic object just for this drop down lists – outside of the factory, inside the controller:

1         public JsonResult GetDoctrinesForAnIssue(int issueId)
2         {
3             IDoctrineFactory doctrineFactory = new DoctrineFactory();
4             var query = (from doctrine in doctrineFactory.GetDoctrines(issueId)
5                          select new { doctrine.DoctrineId, doctrine.DoctrineName }).ToList();
6 
7             return Json(query);
8         }
9  

Add a little LINQ and boom goes the dynamite…