Issue with dynamic 'OrderBy' and AutoMapper inherited types

[onionhammer]

Similar description to #11453, but with EF Core 3.1 preview;

If I generate an 'OrderBy' using a dynamic expression, I receive an 'InvalidOperationException',
"...could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either "

If I use the equivalent non-generated code, the query executes fine.

OrderByExpression() is defined here, and works for simple cases, but when combined with

Steps to reproduce

1. Ensure docker is installed
2. Run the following

git clone https://github.com/onionhammer/ReproduceEFCoreIssue.git
cd ReproduceEFCoreIssue
dotnet test

Error Message:
System.InvalidOperationException : The LINQ expression 'OrderByDescending<Classroom, int>(
source: DbSet,
keySelector: (c) => new ClassroomViewModel{
Classroom = c.Name,
NumStudents = Count(Where(
source: DbSet,
predicate: (s) => Property<Nullable>(c, "Id") != null && Property<Nullable>(c, "Id") == Property<Nullable>(s, "ClassroomId"))),
TeacherNames = Select<Teacher, string>(
source: Where(
source: DbSet,
predicate: (t) => Property<Nullable>(c, "Id") != null && Property<Nullable>(c, "Id") == Property<Nullable>(t, "ClassroomId")),
selector: (t) => t.Name)
}
.NumStudents)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
Stack Trace:

Further technical details

EF Core version:
Database provider: (e.g. Microsoft.EntityFrameworkCore.SqlServer)
Target framework: (e.g. .NET Core 3.0)
Operating system:
IDE: (e.g. Visual Studio 2019 16.3)

[smitpatel]

NumStudents likely you have wrong PropertyInfo in your member expression. See #18386

[onionhammer]

@smitpatel Both expressions are the same, I verified the expression tree

[smitpatel]

In the memberExpression p => p.DateAdded, the memberInfo.ReflectedType is different than what compiler generates. So manually generated expression tree is somewhat incorrect that compiler generated. Hence it does not work. Exact duplicate of #18386

[onionhammer]

@smitpatel So since this code worked flawlessly for years in EntityFramework, what is it, in practical terms, about EF Core that doesnt like this code

[smitpatel]

// Form1
new ClassroomViewModel {
    Classroom = c.Name,
    NumStudents = s.Students.Count()
}.NumStudents

// Form2
s.Students.Count()

In order for us to convert Form1 to form2, the MemberInfo inside ClassroomViewModel NumStudents must match MemberInfo used in MemberExpression. In your dynamically generated case they are different so we cannot simplify it. So you get error. Given compiler does not generate that kind of expression tree, it is low priority for us to try to support such scenarios.

[onionhammer]

So in other words, it's an open bug, not a closed bug? but you're saying it's low priority?

These expressions look identical to me, left hand side is defined statically, the right hand side is dynamically constructed, left hand side works, right hand side fails

[smitpatel]

ReflectedType is different in both sides.
It's closed bug because right now we don't think we will ever get time to fix it.

[onionhammer]

@smitpatel thanks, I must be blind :)

If anyone is interested, I've updated the GIST with the working 'ObjectQueryExtensions.cs'

[liuliang-wt]

I also run into this trouble , constructing a dynamic expression is critical to our product. For example, we let user only write basic select query, and we add orderby expression dynamically when he click a list title. If you are not going to fix this, please at least tell me a walk around.

[liuliang-wt]

@onionhammer hi, have you solved this problem?

[smitpatel]

@liuliang-wt - See #18386 (comment)

[onionhammer]

@liuliang-wt Take a look at this:

https://gist.github.com/onionhammer/5137cefeface774af96af905a76593b6

[smitpatel]

We are enabling support for this scenario in #19182