Includes doesn't work with LinqKit AsExpandable - entity-framework

Question

I'm trying to use LinqKit AsExpandable in my EfCore2.0 project and I am running into this problem where the Includes doesn't work.
In attempting to debug this I have downloaded the LinqKit source from github and have replaced the Nuget references in my project with Project references.
When debugging with the LinqKit project, I noticed that the my call to Include wasn't hitting a breakpoint I set on ExpandableQueryOfClass<T>.Include.
I did some further testing and noticed the breakpoint is hit if I first cast to ExpandableQueryOfClass (this is an internal class in LinqKit which I made public, so I can't do the cast if I'm referencing the Nuget package).
Is this a bug in LinqKit or am I doing something wrong?
Here is my test code.
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
using Internal.DAL.Db;
using Internal.Models.Customer;
using LinqKit; // Referencing LinqKit.Microsoft.EntityFrameworkCore
using Xunit;
namespace Internal.EntityFramework.Tests
{
public class UnitTest1
{
private DbContextOptionsBuilder<DataContext> _ctxBuilder =>
new DbContextOptionsBuilder<DataContext>().UseSqlServer(Connection.String);
[Fact]
public async Task SuccessTest()
{
using (var ctx = new DataContext(_ctxBuilder.Options))
{
var query =
(
// this cast is the only difference between the methods
(ExpandableQueryOfClass<Order>)
ctx.Orders
.AsExpandable()
)
.Include(r => r.Customer)
.Take(500);
var responses = await query.ToListAsync();
// this succeeds
Assert.All(responses, r => Assert.NotNull(r.Customer));
}
}
[Fact]
public async Task FailTest()
{
using (var ctx = new DataContext(_ctxBuilder.Options))
{
var query = ctx.Orders
.AsExpandable()
.Include(r => r.Customer)
.Take(500);
var responses = await query.ToListAsync();
// this fails
Assert.All(responses, r => Assert.NotNull(r.Customer));
}
}
}
}
Edit 2018-05-15: There is an open issue on the LinqKit github repository.

Answer 1

I'm not sure whether this is LINQKit or EF Core fault (definitely it's not yours).
For sure it's caused by the EF Core Include / ThenInclude implementations which all include a check for source.Provider is EntityQueryProvider and do nothing in case it's false.
I'm not sure what's the idea of EF Core designers - probably custom query providers to inherit from EntityQueryProvider, but at the same time the class is part of the infrastructure and marked as not supposed to be used.
I also have no idea how LINQKit is planning to address it, but as you noticed the current implementation definitely is broken/not working. At the moment it looks to me more like a WIP.
The only workaround I see at this time is to apply AsExpandable() after includes.