How to Mock an Entity Framework 6 Async Projecting Query

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By leveraging the Testing with async queries section of the Testing with a Mocking Framework article on MSDN, I've been able to create many successfully passing tests.

Here's my test code, which uses NSubstitute for mocks:

var dummyQueryable = locations.AsQueryable();

var mock = Substitute.For<DbSet<Location>, IDbAsyncEnumerable<Location>, IQueryable<Location>>();
((IDbAsyncEnumerable<Location>)mock).GetAsyncEnumerator().Returns(new TestDbAsyncEnumerator<Location>(dummyQueryable.GetEnumerator()));
((IQueryable<Location>)mock).Provider.Returns(new TestDbAsyncQueryProvider<Location>(dummyQueryable.Provider));
((IQueryable<Location>)mock).Expression.Returns(dummyQueryable.Expression);
((IQueryable<Location>)mock).ElementType.Returns(dummyQueryable.ElementType);
((IQueryable<Location>)mock).GetEnumerator().Returns(dummyQueryable.GetEnumerator());
sut.DataContext.Locations = mock;

var result = await sut.Index();

result.Should().BeView();

sut.Index() doesn't do much, but it makes the following query:

await DataContext.Locations
    .GroupBy(l => l.Area)
    .ToListAsync());

This works fine until I add a projection into the query:

await DataContext.Locations
    .GroupBy(l => l.Area)
    .Select(l => new LocationsIndexVM{ Area = l.Key }) // added projection
    .ToListAsync());

which results in this exception:

System.InvalidOperationException
The source IQueryable doesn't implement IDbAsyncEnumerable<LocationsIndexVM>. Only sources that implement IDbAsyncEnumerable can be used for Entity Framework asynchronous operations. For more details see http://ift.tt/1aRsSNY.
   at System.Data.Entity.QueryableExtensions.AsDbAsyncEnumerable(IQueryable`1 source)
   at System.Data.Entity.QueryableExtensions.ToListAsync(IQueryable`1 source)
   at Example.Web.Controllers.HomeController.<Index>d__0.MoveNext() in HomeController.cs: line 25
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Example.Test.Web.Controllers.HomeControllerShould.<TempTest>d__4.MoveNext() in HomeControllerShould.cs: line 71

Can anyone provide an example of what is required to unit test a query that is both async and contains a .Select() projection?

asked 16 secs ago

nikmd23

5,587