如何在Entity Framework中加入对象

Question

So I am converting a old project with ordinary SQL queries to a ORM using the Entity Framework. So I have created database model like this:

So I had this old query which I want to translate to a linq expression

SELECT UGLINK.USERNAME 
FROM GMLINK 
INNER JOIN UGLINK 
ON GMLINK.GROUPID = UGLINK.GROUPID 
WHERE (((GMLINK.MODULEID)=%ID%))

And the problem I have is that I can't figure out how to do a join query using the objects. Instead I have to go though the properties like this (which seems to be working):

// So this is one of the module objects that is located in a listView in the GUI
Module m = ModuleList.selectedItem as Module;

/* Now I want to fetch all the User objects that, 
 * via a group, is connected to a certain module */
var query = context.gmLink
    .Join(context.ugLink,
          gmlink => gmlink.GroupId,
          uglink => uglink.GroupId,
          (gmlink, uglink) => new { gmLink = gmlink, ugLink = uglink })
    .Where(gmlink => gmlink.gmLink.ModuleId == m.ModuleId)
    .Select(x => x.ugLink.User);

So as I said this works, but as you see I kind of have to connect the modules via the link tables properties .GroupId and .ModuleId and so on. Instead I would like to go through the objects created by EF.

I wanted to write a question a bit like this, but can't figure out how to do it, is it at all possible?

var query = context.User
            .Select(u => u.ugLink
                .Select(uglink => uglink.Group.gmLink
                    .Where(gmLink => gmLink.Module == m)));

Answer 1

If you have lazy loading enabled, you do not need to apply specific join notation (you can access the navigation properties directly) - but the queries that are ran against SQL are inefficient (generally the results are returned in a number of different select statements).

My preference is to disable lazy loading on the context, and use .Include() notation to join tables together manually, resulting in generally more efficient queries. .Include() is used to explicitly join entities in Entity Framework.

Join() is misleading, and not appropriate for joining tables in EF.

So, to replicate this statement:

SELECT UGLINK.USERNAME 
FROM GMLINK 
INNER JOIN UGLINK 
ON GMLINK.GROUPID = UGLINK.GROUPID 
WHERE (((GMLINK.MODULEID)=%ID%))

You would use the following:

var query = context.gmLink
    .Include(x => x.Group.gmLink)
    .Where(x => x.ModuleId == myIdVariable)
    .Select(x => new {
        UserName = x.Group.ugLink.UserName
    });

Assuming that your navigation properties are correctly set up. I have not tested this, so I'm not 100% on the syntax.

You should really run SQL profiler while you write and run LINQ to Entity queries against your database, so you can understand what's actually being generated and run against your database. A lot of the time, an EF query may be functioning correctly, but you may experience performance issues when deployed to a production system.

This whitepaper might help you out.

Answer 2

This should be working:

var query = context.gmLink
    .Where(gmlink => gmlink.ModuleId == m.ModuleId)
    .SelectMany(gmlink => gmlink.Group.ugLink)
    .Select(uglink => uglink.User);

It's impossible to filter gmLinks using .Where(gmlink => gmlink.Module == m) in EF, so this comparison needs to be done using identifiers. Another option is .Where(gmlink => gmlink.Module.ModuleId == m.ModuleId)

Answer 3

I haven't tested it, but something like this:

var users = context.User
    .Where(x => x.ugLink
        .Any(y => context.gmLink
                    .Where(z => z.ModuleId == m)
                    .Select(z => z.GroupId)
                    .Contains(y.GroupId)
        )
    )
    .ToList();