c# - Entity Framework 4.1 代码优先和自动映射器问题

Question

考虑这个简单的模型和 View 模型场景:

public class SomeModel
{
    public virtual Company company {get; set;}
    public string name {get; set;}
    public string address {get; set;}

    //some other few tens of properties
}

public class SomeViewModel
{
    public Company company {get; set;}
    public string name {get; set;}
    public string address {get; set;}
    //some other few tens of properties
}

出现的问题是:

我有一个不需要公司的编辑页面，所以我不从数据库中获取它。现在，当提交表单时，我会:

SomeModel destinationModel = someContext.SomeModel.Include("Company").Where( i => i.Id == id) // assume id is available from somewhere.

然后我做一个

Company oldCompany = destinationModel.company; // save it before mapper assigns it null

Mapper.Map(sourceViewModel,destinationModel);

//After this piece of line my company in destinationModel will be null because sourceViewModel's company is null. Great!!
//so I assign old company to it

destinationModel.company = oldCompany;

context.Entry(destinationModel).State = EntityState.Modified;

context.SaveChanges();

问题是，即使我将 oldCompany 分配给我的公司，它在保存更改后在数据库中仍然为空。

注意:

如果我改变这些行:

destinationModel.company = oldCompany;

context.Entry(destinationModel).State = EntityState.Modified;

context.SaveChanges();

对这些:

context.Entry(destinationModel).State = EntityState.Modified;

destinationModel.company = oldCompany;

context.Entry(destinationModel).State = EntityState.Modified;

context.SaveChanges();

请注意，我更改了 2 次状态，效果很好。可能是什么问题？这是 ef 4.1 错误吗？

这是解决该问题的示例控制台应用程序:

using System;
using System.Linq;
using System.Data.Entity;
using System.ComponentModel.DataAnnotations;
using AutoMapper;

namespace Slauma
{
    public class SlaumaContext : DbContext
    {
        public DbSet<Company> Companies { get; set; }
        public DbSet<MyModel> MyModels { get; set; }

        public SlaumaContext()
        {
            this.Configuration.AutoDetectChangesEnabled = true;
            this.Configuration.LazyLoadingEnabled = true;
        }
    }

    public class MyModel
    {
        public int Id { get; set; }
        public string Foo { get; set; }

        [ForeignKey("CompanyId")]
        public virtual Company Company { get; set; }

        public int? CompanyId { get; set; }
    }

    public class Company
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }


    public class MyViewModel
    {
        public string Foo { get; set; }

        public Company Company { get; set; }

        public int? CompanyId { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {

            Database.SetInitializer<SlaumaContext>(new DropCreateDatabaseIfModelChanges<SlaumaContext>());

            SlaumaContext slaumaContext = new SlaumaContext();

            Company company = new Company { Name = "Microsoft" };
            MyModel myModel = new MyModel { Company = company, Foo = "Foo"};

            slaumaContext.Companies.Add(company);
            slaumaContext.MyModels.Add(myModel);
            slaumaContext.SaveChanges();

            Mapper.CreateMap<MyModel, MyViewModel>();
            Mapper.CreateMap<MyViewModel, MyModel>();


            //fetch the company
            MyModel dest = slaumaContext.MyModels.Include("Company").Where( c => c.Id == 1).First(); //hardcoded for demo

            Company oldCompany = dest.Company;

            //creating a viewmodel
            MyViewModel source = new MyViewModel();
            source.Company = null;
            source.CompanyId = null;
            source.Foo = "foo hoo";

            Mapper.Map(source, dest); // company null in dest


            //uncomment this line then only it will work else it won't is this bug?
            //slaumaContext.Entry(dest).State = System.Data.EntityState.Modified; 

            dest.Company = oldCompany;

            slaumaContext.Entry(dest).State = System.Data.EntityState.Modified;
            slaumaContext.SaveChanges();

            Console.ReadKey();

        }
    }
}

Answer 1

默认情况下，Automapper 总是更新从源实例到目标实例的每个属性。因此，如果您不想覆盖您的 Company 属性，那么您必须为您的映射器显式配置它:

Mapper.CreateMap<MyViewModel, MyModel>().ForMember(m => m.Company, c => c.UseDestinationValue());

到目前为止，与 EF 无关。但是，如果您将其与 EF 一起使用，则必须始终如一地使用导航属性 Company 和 CompanyId:您还需要在映射期间使用 CompanyId 的目标值:

Mapper.CreateMap<MyViewModel, MyModel>().ForMember(m => m.CompanyId, c => c.UseDestinationValue());

编辑:但问题不在于您的公司为空，而是在重置后它在数据库中仍然为空。这是因为如果您拥有像“CompanyId”这样的显式 Id 属性，则必须维护它。所以仅仅调用 destinationModel.company = oldCompany; 是不够的，你还需要调用 destinationModel.companyId = oldCompany.Id;

并且因为您从上下文中检索了目标实体，它已经在为您进行更改跟踪，因此无需设置 EntityState.Modified。

编辑:您修改的样本:

Mapper.CreateMap<MyModel, MyViewModel>();
Mapper.CreateMap<MyViewModel, MyModel>();    

//fetch the company 
MyModel dest = slaumaContext.MyModels.Include("Company").Where(c => c.Id == 18).First(); //hardcoded for demo 

var oldCompany = dest.Company;

//creating a viewmodel 
MyViewModel source = new MyViewModel();
source.Company = null;
source.CompanyId = null;
source.Foo = "fdsfdf";

Mapper.Map(source, dest); // company null in dest 

dest.Company = oldCompany;
dest.CompanyId = oldCompany.Id;

slaumaContext.SaveChanges();