Question

I am fairly new to Entity Framework and investigating converting some legacy data access code to using EF. I want to know if the following is possible in EF and if yes how.
Say I have a Customer table like this

CustomerId | ProductId | StartDate | EndDate
--------------------------------------------
100        | 999       | 01/01/2012| null

Say I also load Product data from somewhere else (like an XML file) as a cache of product objects.

public class Customer
{
    public int CustomerId {get;set;}
    public int Product {get;set}
    public DateTime StartDate {get;set;}
    public DateTime? EndDate {get;set;}    

}

public class Product
{
    public int ProductId {get;set;}
    public int Description {get;set}
}

Currently in CustomerDal class the method uses a StoredProc to get a Customer object like this

Customer GetCustomer(int customerId)
{
    // setup connection, command, parameters for SP, loop over datareader
    Customer customer = new Customer();
    customer.CustomerId = rdr.GetInt32(0);
    int productId = rdr.GetInt32(1);
    // ProductCache is a singleton object that has been initialised before
    customer.Product = ProductCache.Instance.GetProduct(productId);
    customer.StartDate = rdr.GetDateTime(2);
    customer.EndDate = rdr.IsDbNull(3) ? (DateTime?)null : rdr.GetDateTime(3);
    return customer;
}

My question is this possible using EF when it materializes the Customer object it sets the Product property not from the DB but by another method, in this case from an in memory cache. Similary when saving a new Customer object it only gets the ProductId from the Products property and saves the value in DB.

Answer 1

If you attach your product instances to the EF context then when loading a Customer the Product property will be automatically filled from memory without a query to database as long as the product that is associated to the customer is already attached.

For example, starting with these entities:

public class Customer
{
    public int Id { get; set; }
    public int ProductId { get; set; }
    public string Name { get; set; }
    public Product Product { get; set; }
}

public class Product
{
    public int Id { get; set; }
    public string Description { get; set; }
}

Products will be available globally, for simplicity, lets make it a static class:

public static class CachedProducts
{
    public static Product[] All
    {
        get
        {
            return new Product[] { new Product { Id = 1, Description = "Foo" } };
        }
    }
}

With this in mind we just need to assure that every EF context starts with all the products attached to it:

public class CustomerContext : DbContext
{
    public CustomerContext()
    {
        // Attach products to context
        Array.ForEach(CachedProducts.All, p => this.Products.Attach(p));
    }
    public DbSet<Customer> Customers { get; set; }
    public DbSet<Product> Products { get; set; }
}

And finally, to make the sample complete and runnable we seed the database, request a customer and print the associated product description:

public class DatabaseInitializer : CreateDatabaseIfNotExists<CustomerContext>
{
    protected override void Seed(CustomerContext context)
    {
        var p = new Product { Id = 1, Description = "Foo" };
        var c = new Customer { Id = 1, Product = p, Name = "John Doe" };

        context.Customers.Add(c);
        context.SaveChanges();
    }
}


class Program
{
    static void Main(string[] args)
    {
        Database.SetInitializer<CustomerContext>(new DatabaseInitializer());

        using (var context = new CustomerContext())
        {
            var customer = context.Customers.Single(c => c.Id == 1);

            Console.WriteLine(customer.Product.Description);
        }
    }
}

If you attach a profiler to SQL Server you will notice that the customer is loaded from database but no query is performed to obtain the product since it is already attached to the context. This works when loading a customer and also when saving a new customer with an associated product.

Disclaimer: I’m not an EF expert so this approach may have some undesired side effects that I’m unable to consider.