FirstOrDefault after custom projection == null cannot be translated

[groege]

Steps to reproduce

var queryA = db.MyEntity.Select(x => new MyEntityModel {
    ModelPropA = x.Children.Where(y => y.Type == Types.A).FirstOrDefault(),
    ModelPropB = x.Children.Where(y => y.Type == Types.B).FirstOrDefault(),
});
var queryB = queryA.Select(x => new MyEntityModel {
    ModelPropA = x.ModelPropA,
});

queryA.ToList() //This is working
queryB.ToList() //Throws System.InvalidOperationException: 'Sequence contains more than one element'

//EDIT:
queryA.Where(x => x.ModelPropA == null).ToList() //Throws the System.InvalidOperationException as well

StackTrace

Further technical details

EF Core version:
Database provider: Npgsql.EntityFrameworkCore.PostgreSQL 3.0.0
Target framework: NET Core 3.0
Operating system: WIN10
IDE: Visual Studio 2019 16.4.0 Preview 1.0

[ajcvickers]

@groege Please post a small, runnable project/solution or complete code listing that demonstrates the issue. This will ensure that we can reproduce what you are seeing.

[groege]

ExceptionRepro.zip

So I apologize - queryB actually works - I did have an error in my code (Which I found by doing the repro).

But queryA.Where(x => x.ModelPropA == null).ToList() is not working, it sais it cannot be translated.
See queryC in the repro project.

Thanks for your time!!

[smitpatel]

Simpler query

var query = db.Blogs.Where(e => db.Blogs.Select(b => new { b.Id }).FirstOrDefault() == null).ToList();

[groege]

@smitpatel thanks!
Though that query doesn't seem simpler to me ^^ - looks convoluted to me (compared to just using a normal where clause)

[smitpatel]

@groege - Your normal where clause has custom projection preceding it. The simpler query is for repro and not a work-around.

[smitpatel]

This is anonymous type comparison.
Duplicate of #14672

[smitpatel]

Notes:

• If the result type of FirstOrDefault is non-nullable type then == null is always false (& inverse for != null) Compiler generates error
• For nullable result type result of OrDefault family will be null only if the sequence is null. So we can rewrite this check to Any on sequence itself. For != null check, SingleOrDefault could twist things but following our philosophy or server eval, we treat it same as FirstOrDefault in subquery.
• This could also help us in translating entity equality with subquery & composite key (if we are not doing already)

[smitpatel]

For nullable result type result of OrDefault family will be null only if the sequence is null. So we can rewrite this check to Any on sequence itself. For != null check, SingleOrDefault could twist things but following our philosophy or server eval, we treat it same as FirstOrDefault in subquery.

This is only true if the projection type is non-nullable hence null will produced from empty sequence only. If the result type is nullable then a non-empty sequence can also give null.

[smitpatel]

Re-activating as we didn't really fix this issue.

[smitpatel]

@ajcvickers - While this is not really patch issue, it is getting fixed in 6.0 branch as fixing bug for #26744 Not sure how to set milestone on this one now.