entity framework 4.1 code first and auto mapper issue

Question

Consider this simple Model and ViewModel scenario:

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
}

Problem that occurs is:

I have a edit page where company is not needed so I do not fetch it from database. Now when the form is submitted I do:

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

Then I do a

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();

And problem is even when I assign oldCompany to my company it is still null in database after savechanges.

Note:

If I change these lines:

destinationModel.company = oldCompany;

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

context.SaveChanges();

to these:

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

destinationModel.company = oldCompany;

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

context.SaveChanges();

Notice I change the state 2 times, it works fine. What can be the issue? Is this a ef 4.1 bug?

This is a sample console application to address the issue:

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

The second EDIT in @nemesv's answer or the finetuning of AutoMapper is the way to go in my opinion. You should accept his answer. I only add an explanation why your code doesn't work (but your code with setting the state twice does). First of all, the problem has nothing to do with AutoMapper, you will get the same behaviour when you set the properties manually.

The important thing to know is that setting the state ( Entry(dest).State = EntityState.Modified ) does not only set some internal flag in the context but the property setter for State calls actually some complex methods, especially it calls DbContext.ChangeTracker.DetectChanges() (if you don't disable AutoDetectChangesEnabled).

So, what happens in the first case:

// ...
Mapper.Map(source, dest);
dest.Company = oldCompany;

// at this point the state of dest EF knows about is still the state
// when you loaded the entity from the context because you are not working
// with change tracking proxies, so the values are at this point:
// dest.CompanyId = null    <- this changed compared to original value
// dest.Company = company   <- this did NOT change compared to original value

// The next line will call DetectChanges() internally: EF will compare the
// current property values of dest with the snapshot of the values it had
// when you loaded the entity
slaumaContext.Entry(dest).State = System.Data.EntityState.Modified;

// So what did EF detect:
// dest.Company didn't change, but dest.CompanyId did!
// So, it assumes that you have set the FK property to null and want
// to null out the relationship. As a consequence, EF also sets dest.Company
// to null at this point and later saves null to the DB

What happens in the second case:

// ...
Mapper.Map(source, dest);

// Again in the next line DetectChanges() is called, but now
// dest.Company is null. So EF will detect a change of the navigation property
// compared to the original state
slaumaContext.Entry(dest).State = System.Data.EntityState.Modified;

dest.Company = oldCompany;

// Now DetectChanges() will find that dest.Company has changed again
// compared to the last call of DetectChanges. As a consequence it will
// set dest.CompanyId to the correct value of dest.Company
slaumaContext.Entry(dest).State = System.Data.EntityState.Modified;

// dest.Company and dest.CompanyId will have the old values now
// and SaveChanges() doesn't null out the relationship

So, this is actually normal change tracking behaviour and not a bug in EF.

One thing I find disturbing is that you have a ViewModel which apparently has properties you are not using in the view. If your ViewModel wouldn't have Company and CompanyId all the trouble would disappear. (Or configure at least AutoMapper to not map these properties, as shown by @nemesv.)

Answer 2

Automapper always updates every property from the source instance to the destination instance by default. So if you don't want your Company property overwritten then you have to explicitly configure this for your mapper:

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

So far nothing EF related. But if you are using this with EF you have to use your navigation property Company and the CompanyId consistently: you also need to use the destination value for CompanyId during mapping:

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

EDIT: But the problem is not that your Company is null but after resetting it is still null in the DB. And this caused by the fact that if you are having an explicit Id property like 'CompanyId' you have to mantain it. So it is not enough to call destinationModel.company = oldCompany; you also need to call destinationModel.companyId = oldCompany.Id;

And because you retrieved your dest entity from the context it's already doing the change tracking for you therefore there is no need set EntityState.Modified.

EDIT: Your modified sample:

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();