automatically expand related entity with OData controller - asp.net-web-api2

Question

I have these classes:
public class Items
{
[Key]
public Guid Id { get; set; }
public string ItemCode { get; set; }
public decimal SalesPriceExcl { get; set; }
public decimal SalesPriceIncl { get; set; }
public virtual ICollection<ItemPrice> SalesPrices { get; set; }
public Items()
{
SalesPrices = new HashSet<App4Sales_ItemPrice>();
}
}
public class ItemPrice
{
[Key, Column(Order = 0), ForeignKey("Items")]
public Guid Id { get; set; }
public virtual Items Items { get; set; }
[Key, Column(Order=1)]
public Guid PriceList { get; set; }
public decimal PriceExcl { get; set; }
public decimal PriceIncl { get; set; }
public decimal VatPercentage { get; set; }
}
I want to query the Items and automatically get the ItemPrice collection.
I've created an OData V3 controller:
// GET: odata/Items
//[Queryable]
public IQueryable<Items> GetItems(ODataQueryOptions opts)
{
SelectExpandQueryOption expandOpts = new SelectExpandQueryOption(null, "SalesPrices", opts.Context);
Request.SetSelectExpandClause(expandOpts.SelectExpandClause);
return expandOpts.ApplyTo(db.Items.AsQueryable(), new ODataQuerySettings()) as IQueryable<Items>;
}
But I get the error:
"Cannot serialize null feed"
Yes, some Items have no ItemPrice list.
Can I get past this error, or can I do something different?
Kind regards
Jeroen
I found the underlying error is:
Unable to cast object of type
'System.Data.Entity.Infrastructure.DbQuery1[System.Web.Http.OData.Query.Expressions.SelectExpandBinder+SelectAllAndExpand1[.Models.Items]]'
to type '.Models.Items'.

Answer 1

I've solved it after I came across this post: http://www.jauernig-it.de/intercepting-and-post-processing-odata-queries-on-the-server/
This is my controller now:
SelectExpandQueryOption expandOpts = new SelectExpandQueryOption(null, "SalesPrices", opts.Context);
Request.SetSelectExpandClause(expandOpts.SelectExpandClause);
var result = expandOpts.ApplyTo(db.Items.AsQueryable(), new ODataQuerySettings());
var resultList = new List<Items>();
foreach (var item in result)
{
if (item is Items)
{
resultList.Add((Items)item);
}
else if (item.GetType().Name == "SelectAllAndExpand`1")
{
var entityProperty = item.GetType().GetProperty("Instance");
resultList.Add((Items)entityProperty.GetValue(item));
}
}
return resultList.AsQueryable();
Jeroen

Answer 2

GetItems([FromODataUri] ODataQueryOptions queryOptions)

Answer 3

expanding on Jeroen's post. Anytime a select or expand is involved, OData wraps the results in a SelectAll or SelectSome object; so, we need to unwrap the values rather than do an direct cast.
public static class ODataQueryOptionsExtensions
{
public static IEnumerable<T> ApplyODataOptions<T>(this IQueryable<T> query, ODataQueryOptions options) where T : class, new()
{
if (options == null)
{
return query;
}
var queryable = options.ApplyTo(query);
if (queryable is IQueryable<T> queriableEntity)
{
return queriableEntity.AsEnumerable();
}
return UnwrapAll<T>(queryable).ToList();
}
public static IEnumerable<T> UnwrapAll<T>(this IQueryable queryable) where T : class, new()
{
foreach (var item in queryable)
{
yield return Unwrap<T>(item);
}
}
public static T Unwrap<T>(object item) where T : class, new()
{
var instanceProp = item.GetType().GetProperty("Instance");
var value = (T)instanceProp.GetValue(item);
if (value != null)
{
return value;
}
value = new T();
var containerProp = item.GetType().GetProperty("Container");
var container = containerProp.GetValue(item);
if (container == null)
{
return (T)null;
}
var containerType = container.GetType();
var containerItem = container;
var allNull = true;
for (var i = 0; containerItem != null; i++)
{
var containerItemType = containerItem.GetType();
var containerItemValue = containerItemType.GetProperty("Value").GetValue(containerItem);
if (containerItemValue == null)
{
containerItem = containerType.GetProperty($"Next{i}")?.GetValue(container);
continue;
}
var containerItemName = containerItemType.GetProperty("Name").GetValue(containerItem) as string;
var expandedProp = typeof(T).GetProperty(containerItemName);
if (expandedProp.SetMethod == null)
{
containerItem = containerType.GetProperty($"Next{i}")?.GetValue(container);
continue;
}
if (containerItemValue.GetType() != typeof(string) && containerItemValue is IEnumerable containerValues)
{
var listType = typeof(List<>).MakeGenericType(expandedProp.PropertyType.GenericTypeArguments[0]);
var expandedList = (IList)Activator.CreateInstance(listType);
foreach (var expandedItem in containerValues)
{
var expandedInstanceProp = expandedItem.GetType().GetProperty("Instance");
var expandedValue = expandedInstanceProp.GetValue(expandedItem);
expandedList.Add(expandedValue);
}
expandedProp.SetValue(value, expandedList);
allNull = false;
}
else
{
var expandedInstanceProp = containerItemValue.GetType().GetProperty("Instance");
if (expandedInstanceProp == null)
{
expandedProp.SetValue(value, containerItemValue);
allNull = false;
}
else
{
var expandedValue = expandedInstanceProp.GetValue(containerItemValue);
if (expandedValue != null)
{
expandedProp.SetValue(value, expandedValue);
allNull = false;
}
else
{
var t = containerItemValue.GetType().GenericTypeArguments[0];
var wrapInfo = typeof(ODataQueryOptionsExtensions).GetMethod(nameof(Unwrap));
var wrapT = wrapInfo.MakeGenericMethod(t);
expandedValue = wrapT.Invoke(null, new[] { containerItemValue });
if (expandedValue != null)
{
expandedProp.SetValue(value, expandedValue);
allNull = false;
}
}
}
}
containerItem = containerType.GetProperty($"Next{i}")?.GetValue(container);
}
if (allNull)
{
return (T)null;
}
return value;
}
}