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First encounter with OData

After installing VS2010 Professional I could not resist my temptation to start spelunking the newer features available to me. I have seen a couple of videos about OData in Mix10 and I thought it would be the easiest to begin with. I didn’t know the very first encounter with creature of this type would demystify a bug in it. I opened up my solution which comprises of various test projects. I added a new Console Application Project targeting the DotNET4ClientFramework. I added reference to the Mix10Live OData Feed and followed as was demonstrated by presenters in the mix video.

Following is the code

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using ODataTest.Mix10Live;

namespace ODataTest

{

    class Program

    {

        static void Main(string[] args)

        {

            EventEntities eventEntities = new EventEntities(new Uri("http://api.visitmix.com/OData.svc"));

            try

            {

                var query = eventEntities.Speakers.Where(x => x.Sessions.Count > 0).Take(10);

                query.ToList().ForEach(x =>

                    {

                        Console.WriteLine(new String('-', 10));

                        Console.WriteLine(x.SpeakerDisplayName);

                        Console.WriteLine(x.Bio);

                        x.Sessions.ToList().ForEach(y => Console.WriteLine("Session: {0}", y.Title));

                        Console.WriteLine(new String('-', 10));

                    });

            }

            catch (Exception ex)

            {

                Console.WriteLine(ex);

            }

        }

    }

}

The LINQ Query generates following URL:

http://api.visitmix.com/OData.svc/Speakers()?$filter=Sessions/Count gt 0&$top=10

Following the URL leads to an xml output which gives following error

An error occurred while processing this request. Property 'System.Nullable`1[System.DateTime] PublishedDate' is not defined for type 'System.Data.Objects.DataClasses.EntityCollection`1[api.visitmix.com.Session]' System.ArgumentException at System.Linq.Expressions.Expression.Property(Expression expression, PropertyInfo property) at System.Linq.Expressions.Expression.MakeMemberAccess(Expression expression, MemberInfo member) at System.Data.Services.ExpressionVisitor.VisitMemberAccess(MemberExpression m) at System.Data.Services.ExpressionVisitor.Visit(Expression exp) at System.Data.Services.ExpressionVisitor.VisitBinary(BinaryExpression b) at System.Data.Services.ExpressionVisitor.Visit(Expression exp) at System.Data.Services.RequestQueryProcessor.ComposePropertyNavigation(Expression expression, LambdaExpression filterLambda, Boolean propagateNull) at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseMemberAccess(Expression instance) at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseIdentifier() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParsePrimaryStart(ExpressionLexer l) at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParsePrimary() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseUnary() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseMultiplicative() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseAdditive() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseComparison() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseLogicalAnd() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseLogicalOr() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseExpression() at System.Data.Services.Parsing.RequestQueryParser.ExpressionParser.ParseWhere() at System.Data.Services.Parsing.RequestQueryParser.ParseLambdaForWhere(IDataService service, ResourceSetWrapper setForIt, ResourceType typeForIt, Type queryElementType, String expression) at System.Data.Services.Parsing.RequestQueryParser.Where(IDataService service, ResourceSetWrapper setForIt, ResourceType typeForIt, IQueryable source, String predicate) at System.Data.Services.RequestQueryProcessor.ProcessFilter() at System.Data.Services.RequestQueryProcessor.ProcessQuery() at System.Data.Services.RequestQueryProcessor.ProcessQuery(IDataService service, RequestDescription description) at System.Data.Services.RequestUriProcessor.ProcessRequestUri(Uri absoluteRequestUri, IDataService service) at System.Data.Services.DataService`1.ProcessIncomingRequestUri() at System.Data.Services.DataService`1.HandleRequest()

This looks strange as I did not use PublishedDate in my code and the service reference was generated entirely by the IDE. On Twitter, I was told that the “Count” filter is invalid. Now, since the query is generated by Linq2OData, the onus is on either OData or on Linq2OData to provide compatible query or generate error while generating the query itself.

I played around a little more and modified the LINQ query as follows

var query = eventEntities.Speakers.Where(x => !String.IsNullOrWhiteSpace(x.Twitter)).Take(10);

This time it gives me following error

{Error translating Linq expression to URI: The expression Not(IsNullOrWhiteSpace([10007].Twitter)) is not supported.}

So, this is established that the LINQ does validate the linq expression before generating the OData uri and also, not all LINQ operators are compatible with OData operators.

I would like to know where the bug resides, in OData or LINQ.