Using Entity Framework 4.0 with Code-First and POCO: How to Get Parent Object with All its Children?

Question

I'm new to EF 4.0, so maybe this is an easy question. I've got VS2010 RC and the latest EF CTP. I'm trying to implement the "Foreign Keys" code-first example on the EF Team's Design Blog, http://blogs.msdn.com/efdesign/archive/2009/10/12/code-only-further-enhancements.aspx.

public class Customer
{
   public int Id { get; set; 
   public string CustomerDescription { get; set; 
   public IList<PurchaseOrder> PurchaseOrders { get; set; }
}

public class PurchaseOrder
{
   public int Id { get; set; }
   public int CustomerId { get; set; }
   public Customer Customer { get; set; }
   public DateTime DateReceived { get; set; }
}

public class MyContext : ObjectContext
{
   public RepositoryContext(EntityConnection connection) : base(connection){}
   public IObjectSet<Customer> Customers { get {return base.CreateObjectSet<Customer>();} }
}

I use a ContextBuilder to configure MyContext:

{
   var builder = new ContextBuilder<MyContext>();

   var customerConfig = _builder.Entity<Customer>();
   customerConfig.Property(c => c.Id).IsIdentity();

   var poConfig = _builder.Entity<PurchaseOrder>();
   poConfig.Property(po => po.Id).IsIdentity();

   poConfig.Relationship(po => po.Customer)
      .FromProperty(c => c.PurchaseOrders)
      .HasConstraint((po, c) => po.CustomerId == c.Id);

   ...
}

This works correctly when I'm adding new Customers, but not when I try to retrieve existing Customers. This code successfully saves a new Customer and all its child PurchaseOrders:

using (var context = builder.Create(connection))
{
   context.Customers.AddObject(customer);
   context.SaveChanges();
}

But this code only retrieves Customer objects; their PurchaseOrders lists are always empty.

   using (var context = _builder.Create(_conn))
   {
      var customers = context.Customers.ToList();
   }

What else do I need to do to the ContextBuilder to make MyContext always retrieve all the PurchaseOrders with each Customer?

Answer 1

Well the solution turned out to be simple, as I suspected it might. I called the context.LoadProperty() method for each individual customer:

using (var context = _builder.Create(_conn))
{
    var customers = context.Customers.ToList();
    foreach (var customer in customers)
    {
        context.LoadProperty<Customer>(customer, c => c.PurchaseOrders);
    }
    return customers;
}

Answer 2

I'm working with the released version of 4.0/VS 2010/EF and I can't seem to find the namespace where the ContextBuilder is stored. In the CTP examples I've seen it is in 'Microsoft.Data.Objects', but I can't seem to find that reference. I had no issues creating a Model-First example, but I'm stuck on the Code-First example I'm working with...any help is greatly appreciated!

Answer 3

You could also use:

var customers = context.Customers.Include("PurchaseOrders").ToList();

Or enable LazyLoading in the ContextOptions :

context.ContextOptions.LazyLoadingEnabled = true;

Just be careful with deferred loading if you are serializing the objects or you may end up querying the entire database.