Linq查询返回复杂类型

Question

I have a pretty standard model for Users and Roles:

+-------+   +-----------+   +-------+
| Users |   | UserRoles |   | Roles |
+-------+   +-----------+   +-------+
| ID (P)| < | UserID (P)|   | ID (P)|
| ...   |   | RoleID (P)| > | ...   |
+-------+   +-----------+   +-------+

I am using the Entitiy framework as my ORM and I am trying to construct - in a single Linq query - my ViewModel, definied below:

public class RoleDetail {
  public class RoleUser {
    public int ID { get; set; }
    public string Username { get; set; }
  }
  public int ID { get; set; }
  public string Rolename { get; set; }
  public bool Active { get; set; }
  public IEnumerable<RoleUser> Users { get; set; }
}

I figured that the following query should work:

var query = Context.Roles
                   .Include("Users")
                   .Where(r => r.ID == id)
                   .Select(r => new RoleDetail() {
                      ID = r.ID,
                      Rolename = r.Rolename,
                      Active = r.Active,
                      Users = r.Users
                               .Select(u => new RoleDetail.RoleUser() {
                                  ID = u.ID,
                                  Username = u.Username
                                })
                                .ToList()
                    })
                   .FirstOrDefault();

However, this throws the following error:

LINQ to Entities does not recognize the method 'System.Collections.Generic.List [RolesRepository+RoleDetail+RoleUser] ToList[RoleUser] (System.Collections.Generic.IEnumerable[RolesRepository+RoleDetail+RoleUser])' method, and this method cannot be translated into a store expression.

I haven't worked with Linq a lot, so I am quite sure that my query is wrong somehow. Can someone show me where my query is going wrong and if there is a better way to accompish what I'm trying to achieve?

Thank you in advance!

Answer 1

Little experience with linq-to-entities, so this is a shot in the dark:

Why do you need .ToList()?

.Select() already returns an IEnumerable, so it should work without.

ToList() returns a List, not an IList or similar interface, but a concrete c# class. So I'd imagine that Linq to Entities cannot translate that into its own behind the scenes syntax

Does this work?

       var query = Context.Roles
               .Include("Users")
               .Where(r => r.ID == id)
               .Select(r => new RoleDetail() {
                  ID = r.ID,
                  Rolename = r.Rolename,
                  Active = r.Active,
                  Users = r.Users
                           .Select(u => new RoleDetail.RoleUser() {
                              ID = u.ID,
                              Username = u.Username
                            })
                            // .ToList()
                })
               .FirstOrDefault();

Answer 2

Everything in your LINQ query that gets sent off to the EF LINQ provider needs to be an expression that L2E can translate into SQL directly. It's up to you to manually separate the parts that EF can translate, from the parts that have to be done in LINQ 2 Objects.

The execution happens at the point where the query has to be actually enumerated, which in your case is the FirstOrDefault call. You can force the query to be executed earlier by calling IQueryable.AsEnumerable() ; anything after that is processed by LINQ 2 Objects and can pretty much do whatever you want.

In your case, the only non-translatable aspect of your lambda expression is the call to ToList , which has no database equivalent, so you could simply not call that; the downside to this is that your Users property will be set up some arbitrary internal class that happens to be the result of IQueryable.Select in the EF's implementation. That may or may not be a problem for you, depending on what you defined RoleDetail.Users as and what you do with it.

The alternative, and the more generally-applicable solution when you have to process an expression that EF doesn't like, is to insert the AsEnumerable call early:

var query = Context.Roles
           .Include("Users")
           .Where(r => r.ID == id)
           .AsEnumerable()
           .Select(r => new RoleDetail() {
              ID = r.ID,
              Rolename = r.Rolename,
              Active = r.Active,
              Users = r.Users
                       .Select(u => new RoleDetail.RoleUser() {
                          ID = u.ID,
                          Username = u.Username
                        })
                        // .ToList()
            })
           .FirstOrDefault();

Answer 3

Looks like it's trying to execute ToList at the database and not knowing how. I would try changing it to ToArray , or failing that, just leaving it as IEnumerable.