实体框架-加载特定的外键数据

Question

I have been trying to make it work but I don't know what I am missing here.

My models are:

public class User
{
    public int id { get; set; }
    public string username { get; set; }

    public virtual List<UserMeta> usermeta { get; set; }

    public User()
    {
        this.usermeta = new List<UserMeta>();
    }

}

public class UserMeta
{
    public int id { get; set; }
    public string metakey { get; set; }
    public string value { get; set; }

    public virtual User user { get; set; }
}

I want to select metavalue for metakey = 'first_name'

Currently this is what I have and its working:

var user = db.Users.Include("UserMeta");

foreach (var item in user)
{
    Console.WriteLine(item.username);

    foreach (var item2 in item.usermeta)
    {
        // check item2.metakey for 'first_name'   
    }
}

But it won't work like this:

var user = from u in db.Users
           select u;

foreach (var item in user)
{
    foreach (var item2 in item.usermeta)
    {
    }         
}

What is wrong in the second code snippet. Also is there a better way of loading these?

Answer 1

In the second code snippet you do not include the UserMeta table.

Try this:

var user = (from u in db.Users select u).Include("UserMeta");

Answer 2

I test this code in Console Application with EF 6.1.3 and no Problem (It is unlikely that the version of ef is problem), please test this code or compare with your code(your code is incomplete).

class Program
{
    static void Main(string[] args)
    {
        using (var db = new ApplicationContext())
        {
            var user = from u in db.Users select u;

            foreach (var item in user)
            {
                foreach (var item2 in item.usermeta)
                {
                    Console.WriteLine(string.Format($"{item2.metakey}:{item2.value}"));
                }
            }
        }

        Console.ReadKey();
    }
}