No nested results in Entity Framework Core [duplicate]

Question

This question already has an answer here:

• EF Core returns null relations until direct access 1 answer

i have a strange behavior in C# with EF It's a .NET Core project with EF Core 1.1.0

"Microsoft.EntityFrameworkCore.SqlServer": "1.1.0",
"Microsoft.EntityFrameworkCore.Tools": "1.1.0-preview4-final",

I've created two Models, "User" and "Group"

 public class User
 {
    public int Id { get; set; }
    public string name { get; set; }
    public string lastName { get; set; }

    public List<Group> Groups { get; set; }
 }

 public class Group
 {
    public int Id { get; set; }
    public string groupName { get; set; }
    public virtual User User { get; set; }
 }

The result schould give me a User with a list of his groups.

Now the strange part:

Result comes back from DB, i look into allUser, Groups are Null

I take a look into the context to see the Groups

Groups are filled:

Now i look again into allUser Result and magically the Group is filled inside every User item.

Groups are filled inside User:

I really appreciate any help!!

Answer 1

Looks like you are lazy loading the entities.

You can eager load them by using .Include.

 var allUsers = context.Users.Include(user => user.Groups).ToList();

This approach loads the first entity (Users) as well as the related entities as part of the query (Groups).

Answer 2

It's not magic the DbContext won't download entities from the DB until you not tirgger that. When you clicked on the context.Groups Results View in debugging, the DbContext gets all the Groups from the DB so after that it can associate to the users.

I think you are searching for the Include method where you can get the users with the associated groups in one query like this:

var allUsers = context.Users.Include(u => u.Groups).ToList();

Answer 3

This behavior comes from the laziness of loading entities.

By default, you have to manually load each of sub-entity. It is called eager-loading.

In order to eager-load your entities, you have to use the .Include() method on each of your navigation properties.

e.g. context.User.Include(x => x.Groups)

The Include method generates a new SQL statement to retrieve your Groups from database.

Then you can use the ToList() method to force executing your query.