Azure Mobile Service Lookupasync load navigation properties - azure

Question

I have a Place data_object in the service side which contains a navigation property Roads:
public class Place : EntityData
{
...
public List<Road> Roads { get; set; }
}
And now on the client side, I want to get a Place object using its id, but the navigation property Roads just won't load. Is there any parameter or attribute I can add to make it work?
My code for it:
var roadList = await App.MobileService.GetTable<Place>()
.LookupAsync(placeId);

Answer 1

Since loading navigation properties in EF requires a JOIN operation in the database (which is expensive), by default they are not loaded, as you noticed. If you want them to be loaded, you need to request that from the client, by sending the $expand=<propertyName> query string parameter.
There are two ways of implementing this: in the server and in the client. If you want to do that in the server, you can implement an action filter which will modify the client request and add that query string parameter. You can do that by using the filter below:
[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
class ExpandPropertyAttribute : ActionFilterAttribute
{
string propertyName;
public ExpandPropertyAttribute(string propertyName)
{
this.propertyName = propertyName;
}
public override void OnActionExecuting(HttpActionContext actionContext)
{
base.OnActionExecuting(actionContext);
var uriBuilder = new UriBuilder(actionContext.Request.RequestUri);
var queryParams = uriBuilder.Query.TrimStart('?').Split(new[] { '&' }, StringSplitOptions.RemoveEmptyEntries).ToList();
int expandIndex = -1;
for (var i = 0; i < queryParams.Count; i++)
{
if (queryParams[i].StartsWith("$expand", StringComparison.Ordinal))
{
expandIndex = i;
break;
}
}
if (expandIndex < 0)
{
queryParams.Add("$expand=" + this.propertyName);
}
else
{
queryParams[expandIndex] = queryParams[expandIndex] + "," + propertyName;
}
uriBuilder.Query = string.Join("&", queryParams);
actionContext.Request.RequestUri = uriBuilder.Uri;
}
}
And then you can decorate your method with that attribute:
[ExpandProperty("Roads")]
public SingleItem<Place> GetPlace(string id) {
return base.Lookup(id);
}
Another way to implement this is to change the client-side code to send that header. Currently the overload of LookupAsync (and all other CRUD operations) that takes additional query string parameters cannot be used to add the $expand parameter (or any other $-* parameter), so you need to use a handler for that. For example, this is one such a handler:
class MyExpandPropertyHandler : DelegatingHandler
{
string tableName
string propertyName;
public MyExpandPropertyHandler(string tableName, string propertyName)
{
this.tableName = tableName;
this.propertyName = propertyName;
}
protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
if (request.Method.Method == HttpMethod.Get.Method &&
request.RequestUri.PathAndQuery.StartsWith("/tables/" + tableName, StringComparison.OrdinalIgnoreCase))
{
UriBuilder builder = new UriBuilder(request.RequestUri);
string query = builder.Query;
if (!query.Contains("$expand"))
{
if (string.IsNullOrEmpty(query))
{
query = "";
}
else
{
query = query + "&";
}
query = query + "$expand=" + propertyName;
builder.Query = query.TrimStart('?');
request.RequestUri = builder.Uri;
}
}
return await base.SendAsync(request, cancellationToken);
return result;
}
}
And you'd use the handler by creating a new instance of MobileServiceClient:
var expandedClient = new MobileServiceClient(
App.MobileService.ApplicationUrl,
App.MobileService.ApplicationKey,
new MyExpandPropertyHandler("Place", "Roads"));
var roadList = await App.MobileService.GetTable<Place>()
.LookupAsync(placeId);