Adding and updating entities with Entity Framework

Question

In my last project I have used Entity Framework 5 Code First. I completed my project but had a lot of pain during the development process.

I tried to explain my pain below:

I had several data classes in my data access logic layer like Product, ProductCategory, Order, Company, Manufacturer etc... Each class has some references to some other classes. For example, a Product instance has a ProductCategory property.

Inside Add and Update methods of my Data Access Object classes I set the states of each property (other than the primitive types) to Unchanged or Modified in the context. Below is some part of an update method of some dao class:

context.Entry(entity).State = System.Data.EntityState.Modified;
if (entity.Category != null)
    context.Entry(entity.Category).State = System.Data.EntityState.Unchanged;

if (entity.Manufacturer != null)
    context.Entry(entity.Manufacturer).State = System.Data.EntityState.Unchanged;

foreach (var specificationDefinition in entity.SpecificationDefinitions)
{
    context.Entry(specificationDefinition).State = System.Data.EntityState.Unchanged;
    foreach (var specificationValue in specificationDefinition.Values)
    {
        context.Entry(specificationValue).State = System.Data.EntityState.Unchanged;
    }
}

This code goes like this. Some of my add or update methods are about 30 lines of code. I think I am doing something wrong, adding or updating an entity should not be this much pain but when I don't set the states of the objects I either get exception or duplicate entries in the database. Do I really have to set the state of each property that maps to database?

Can you add the code in which you create the data? If your references in your classes are correct, EF should only create all instances once. — Nullius, Jul 22 '13 at 19:49
Yes it creates all instances at once. That is the problem actually. Let me explain the problem again. Say I have an instance of Product which has a ProductType instance inside it. Assume that Product instance is already persisted in the past. When I try to update that Product instance, EF creates a duplicate ProductType instance (which is a case I don't want) if I don't set the state of the ProductType attribute to Unchanged. — Furkan, Jul 23 '13 at 18:57

score 11 · Accepted Answer · answered Jul 26 '13 at 16:01

11

You can replace your code by:

context.Products.Attach(entity);
context.Entry(entity).State = System.Data.EntityState.Modified;

The reason why this is the same (unless the related entities were already attached to the context in another state than Unchanged before) is that Attach puts entity including all related entities in the object graph into the context in state Unchanged. Setting the state to Modified afterwards for entity only changes the state of the product alone (not the related entities) from Unchanged to Modified.

answered Jul 26 '13 at 16:01

Slauma

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This will not work if entity was retrieved from another (not yet disposed) context. You'll get an exception about this. – Michael Logutov Feb 12 '14 at 11:48
@MichaelLogutov: You are right, but I'm almost sure that the scenario in the question is a detached scenario (i.e. an earlier context is already disposed). Otherwise it's much easier anyway since you normally don't have to fiddle around with any entity states at all because change tracking will keep track of the correct states automatically. – Slauma Feb 12 '14 at 12:38

score 3 · Answer 2 · answered Jul 26 '13 at 09:17

Ok, you're just doing something wrong then. In addition to my comment, I've created a sample for your which shows that EF doesn't create duplicates by default.

I have two classes:

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ProductCategory Category { get; set; }
    public decimal Price { get; set; }
}

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

One context:

public class MyContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<ProductCategory> ProductCategories { get; set; }

    public MyContext()
        : base("name=MyContext")
    {
    }

    public MyContext(string nameOrConnectionString)
        : base(nameOrConnectionString)
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        Database.SetInitializer<MyContext>(null);

        // Table mappings
        modelBuilder.Entity<Product>().ToTable("Product");
        modelBuilder.Entity<ProductCategory>().ToTable("ProductCategory");

        base.OnModelCreating(modelBuilder);
    }
}

Then one initializer class (this could inherit from other strategies if you want):

public class InitDb<TContext> : DropCreateDatabaseAlways<TContext>
    where TContext : DbContext
{
}

The main program:

    static void Main(string[] args)
    {
        var prodCat = new ProductCategory()
        {
            Name = "Category 1"
        };

        var prod = new Product()
        {
            Name = "Product 1",
            Category = prodCat,
            Price = 19.95M
        };

        using (var context = new MyContext())
        {
            var initializer = new InitDb<MyContext>();
            initializer.InitializeDatabase(context);

            Console.WriteLine("Adding products and categories to context.");
            context.ProductCategories.Add(prodCat);
            context.Products.Add(prod);

            Console.WriteLine();
            Console.WriteLine("Saving initial context.");
            context.SaveChanges();
            Console.WriteLine("Context saved.");

            Console.WriteLine();
            Console.WriteLine("Changing product details.");
            var initProd = context.Products.Include(x => x.Category).SingleOrDefault(x => x.Id == 1);
            PrintProduct(initProd);
            initProd.Name = "Product 1 modified";
            initProd.Price = 29.95M;
            initProd.Category.Name = "Category 1 modified";
            PrintProduct(initProd);

            Console.WriteLine();
            Console.WriteLine("Saving modified context.");
            context.SaveChanges();
            Console.WriteLine("Context saved.");

            Console.WriteLine();
            Console.WriteLine("Getting modified product from database.");
            var modProd = context.Products.Include(x => x.Category).SingleOrDefault(x => x.Id == 1);
            PrintProduct(modProd);

            Console.WriteLine();
            Console.WriteLine("Finished!");
            Console.ReadKey();
        }


    }

    static void PrintProduct(Product prod)
    {
        Console.WriteLine(new string('-', 10));
        Console.WriteLine("Id      : {0}", prod.Id);
        Console.WriteLine("Name    : {0}", prod.Name);
        Console.WriteLine("Price   : {0}", prod.Price);
        Console.WriteLine("CatId   : {0}", prod.Category.Id);
        Console.WriteLine("CatName : {0}", prod.Category.Name);
        Console.WriteLine(new string('-', 10));
    }

This results in the following console output:

Adding products and categories to context.

Saving initial context.
Context saved.

Changing product details.
----------
Id      : 1
Name    : Product 1
Price   : 19,95
CatId   : 1
CatName : Category 1
----------
----------
Id      : 1
Name    : Product 1 modified
Price   : 29,95
CatId   : 1
CatName : Category 1 modified
----------

Saving modified context.
Context saved.

Getting modified product from database.
----------
Id      : 1
Name    : Product 1 modified
Price   : 29,95
CatId   : 1
CatName : Category 1 modified
----------

Finished!

Also, when looking in SQL Server Management Studio, this solution has only created (and updated) one product and one category.

Ofcourse, you should work with repositories to retrieve, update and delete your data and a unit of work. These have been left out of the example.

So, if you do not post any code, we can't help you much further :-)

Adding and updating entities with Entity Framework

2 Answers2

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