如何在DTO中加载DTO？

Question

---

PROBLEM:

I am very new to EF and to LINQ, so please bear with me.

I am trying to create a EF6 model using the database first approach. Simply speaking, I have 2 database tables tblUser and tblMilkMan which have a foreign key relationship on the UserID column.

To avoid cyclic references and to shape the entity data I have created DTO classes for both the models.

I made the MilkManDTO class contain a reference to a UserDTO instance.(This is probably stupid, if so, please guide me to the right way).My aim is to be able to load a milkmen and the related User data

Anyway in my API call, when I try to load a MilkMan by ID, I do not know how to load the related UserDTO . I found examples online on how to load related Entities but not related DTOs.

DB Schema:

Models:

MilkMan Model and DTO:

namespace MilkMan.Models
{
    using System;
    using System.Collections.Generic;

    public partial class tblMilkMan
    {
        public int RecordID { get; set; }
        public int UserID { get; set; }
        public bool IsMyTurn { get; set; }
        public int RoundRobinOrder { get; set; }

        public virtual tblUser tblUser { get; set; }
    }

    public class MilkManDTO
    {
        public int RecordID { get; set; }
        public int UserID { get; set; }
        public bool IsMyTurn { get; set; }
        public int RoundRobinOrder { get; set; }

        public virtual UserDTO User { get; set; }
    }
}

User Model and DTO:

public partial class tblUser
    {
        public tblUser()
        {
            this.tblMilkMen = new HashSet<tblMilkMan>();
        }

        public int UserID { get; set; }
        public string LogonName { get; set; }           
        public string Password { get; set; }           
        public int PasswordExpiresAfter { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        :
        // more fields
        :
        public virtual ICollection<tblMilkMan> tblMilkMen { get; set; }
    }

    public class UserDTO
    {
        public int UserID { get; set; }       
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

Web API Controller Method:

// GET api/MilkMan/5
        [ResponseType(typeof(MilkManDTO))]
        public async Task<IHttpActionResult> GettblMilkMan(int id)
        {
            //tblMilkMan tblmilkman = await db.tblMilkMen.FindAsync(id);

            MilkManDTO milkMan = await db.tblMilkMen.Select(b => new MilkManDTO()
            {
                RecordID = b.RecordID,
                UserID = b.UserID,
                IsMyTurn = b.IsMyTurn,
                RoundRobinOrder = b.RoundRobinOrder,
                User = //???? Error//

            }).SingleOrDefaultAsync(b => b.RecordID == id);


            if (milkMan == null)
            {
                return NotFound();
            }

            return Ok(milkMan);
        }

Answer 1

You can nest a new UserDTO and use the same initialization list technique.

        MilkManDTO milkMan = await db.tblMilkMen.Select(b => new MilkManDTO()
        {
            RecordID = b.RecordID,
            UserID = b.UserID,
            IsMyTurn = b.IsMyTurn,
            RoundRobinOrder = b.RoundRobinOrder,
            User = new UserDTO {
               UserID = b.User.UserID,
               FirstName = b.User.FirstName,
               LastName = b.User.LastName,
            } 

        }).SingleOrDefaultAsync(b => b.RecordID == id);

This code may throw a null reference exception on b.User.UserID if there is not associated User and thus User could be null. You would need to deal with this with either a ?? coalesce, ternary (b.User == null ? "DefaultFirstName" : b.User.FirstName) or omit the entire reference User = (b.User == null ? (UserDTO)null : new UserDTO { ... }) . null's make this kind of thing fun.

With C# 6 we have null reference operator .? that makes this much more succinct.