Entity Framework/WebApi unable to retrieve associated results - asp.net

Question

I am trying to retrieve data as XML using Entity Framework and WebAPI.
All I get is an empty Child container, I have populated two records in OrderItem with OrderId=1 yet still nothing, see <OrderItems/> below:
<ArrayOfOrder xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/WebApplication6.Models">
<Order>
<ID>1</ID>
<OrderItems/>
<Username>Test</Username>
</Order>
</ArrayOfOrder>
My Order model is as follows:
public class Order
{
public Order()
{
this.OrderItems = new HashSet<OrderItem>();
}
public int Id { get; set; }
public string Username { get; set; }
public ICollection<OrderItem> OrderItems { get; set; }
}
My OrderItems model is as follows:
public class OrderItem
{
public int Id { get; set; }
public int OrderId { get; set; }
public int Qty { get; set; }
public Order Order { get; set; }
}
Controller code for GetOrders:
// GET api/Order
public IQueryable<Order> GetOrders()
{
return db.Orders;
}
I am just trying to learn entity framework and WebApi, this part seems to be basic functionality and I can't find any solid reference to this at a basic level. Can anyone explain what I am missing?

Answer 1

After hours of searching I found the solution. Basically I needed to include the sub-dataset. This is done in the controller function:
public IQueryable<Order> GetOrders()
{
return db.Orders.Include(p => p.OrderItems);
}
However this introduces the error:
Object graph for type
'System.Collections.Generic.HashSet`1[[WebApplication7.Models.OrderItem,
WebApplication7, Version=1.0.0.0, Culture=neutral,
PublicKeyToken=null]]' contains cycles and cannot be serialized if
reference tracking is disabled.
Which is fixed for XML by using Datacontracts
Updated OrderItem model:
using System.Runtime.Serialization;
[DataContract]
public class OrderItem
{
[DataMember]
public int Id { get; set; }
[DataMember]
public int OrderId { get; set; }
[DataMember]
public int Qty { get; set; }
public Order Order { get; set; }
}
Note there is no [DataMember] annotation before Order
Now my result is as expected:
<ArrayOfOrder xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/WebApplication7.Models">
<Order>
<Id>1</Id>
<OrderItems>
<OrderItem>
<Id>2</Id>
<OrderId>1</OrderId>
<Qty>231</Qty>
</OrderItem>
<OrderItem>
<Id>4</Id>
<OrderId>1</OrderId>
<Qty>2314</Qty>
</OrderItem>
</OrderItems>
<Username>first</Username>
</Order>
</ArrayOfOrder>