Entity Framework - MySQL - Null Comparison!

Question

Whats wrong with following code? :(

int? parentFolderId = null;
if( this.Request.QueryString ["folderId"] != null )
 parentFolderId = Convert.ToInt32( this.Request.QueryString ["folderId"] );
f = ( from x in this.EntityDataContext.folders
  where x.parent_id == parentFolderId
  select x ).ToList();

It returns nothing! Though there ARE records in database with parent_id as NULL.

However, when I explicitly state NULL; it works!

f = ( from x in this.EntityDataContext.folders
  where x.parent_id == null
  select x ).ToList();

What could be the issue?

PS: I hate working with mysql using Entity Framework .... every damn simple thing has million issues!

Answer 1

You'll probably find there's an issue involving using DBNull or something similar. I would think that in the second case (where you explicitly state "null") that Linq is automatically transforming it to DBNull in the background.

perhaps try something along the lines of:

where x.parent_id == ( parentFolderId == null ? DBNull.Value : parentFolderId )

Hope that puts you on the right track!

Answer 2

I had this kind of problem in sql server and in sql server the generated query looks like "parent_id = null" when you are working on a nullable field. And that query returns nothing even parent_id is null.

The tricky way in here is, you should force EF to create a query like "parent_id is null" and the code I tried in linq was;

if(parentFolderId.HasValue)
{
    f = ( from x in this.EntityDataContext.folders
                where x.parent_id == parentFolderId
                select x ).ToList();
}
else
{
    f = ( from x in this.EntityDataContext.folders
                where !x.parent_id.HasValue
                select x ).ToList();
}

I know this does not seem a perfect way to do this but, this is how I could get rid of that issue.

Answer 3

Long Shot

f = ( from x in this.EntityDataContext.folders
                where ((parentFolderId!=null && x.parent_id == parentFolderId)
                        ||(parentFolderId==null && x.parent_id == null))
                select x ).ToList();

Yeah, this seams wired, and I guess your first example should work just fine with MsSql. Maybe it's time to file a bug to authors of Linq to MySql ?

Answer 4

This is connector bug and I have reported at mysql.