Using projection with nested expressions.

[boppercr]

Is there a way to still use projections with nested expression?

Here's some additionnal information. I have an entity that has one or many DTOs of it. Theses DTOs contain other DTOs from other entities, say a Blog has a Category which in turn has a SubCategory. I created a Mapper that works really well when there's only a few layers that looks like this :

public static class BlogMapper
{
    public static Expression<Func<Blog, BlogDTO>> ToBlogDTO = x =>
        new Blog
        {
            //Other properties
            Category = CategoryMapper.ToCategoryDTO.Compile.Invoke(x.Category);
        }
}

public static class CategoryMapper
{
    public static Expression<Func<Category, CategoryDTO>> ToCategoryDTO = x =>
        new Category
        {
            //Other properties
            SubCategory = SubCategoryMapper.ToSubCategoryDTO.Compile.Invoke(x.SubCategory);
        }
}

public static class SubCategoryMapper
{
    public static Expression<Func<SubCategory, SubCategoryDTO>> ToSubCategoryDTO = x =>
        new SubCategory
        {
            //Other properties
        }
}

Then I can use it like the following :

_context.Blogs.Select(BlogMapper.ToBlogDTO).ToList();

The issue arises because the query created by EF Core doesn't contain the SubCategory table and therefor its value is null when trying to map.

It feels like the projection stops at a certain level and I'm not sure I understand why.

The main thing is that I wouldn't want the BlogMapper to take care of the Category and SubCategory mappings since I know I'll already have dedicated mappers for this exact purpose. I'm not gonna lie, I feel like I'm either missing something or that it "doesn't make sense" to go at it like that but I can't seem to find the alternative.

I've tried an approach with simple constructors and they won't work with projection as well. I even tried to create an ExpressionVisitor to "flatten" the resulting expression somehow, but it didn't work as well. I can provide the code if need be, but like I said, I feel like I'm gone too far in the wrong direction.

Include provider and version information

EF Core version: 8.0.10
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 8.0

[roji]

This issue is lacking enough information for us to be able to fully understand what is happening. Please attach a small, runnable project or post a small, runnable code listing that reproduces what you are seeing so that we can investigate.

[boppercr]

Sorry for the delay, I was away for a little time.

I added a small project to help understand better the issue.
efcore-issue-34866.zip

Additionnal information

You can use the efcore-issue-34866.http file to test the API. The /current endpoint
gives you the current behavior of EF Core when using expressions in the mapping.

You can see that the Expression<Func<Data, DTO>> is correctly translated to SQL but only to a single level.

The /expected endpoint gives you the expected behavior of EF Core when using expressions in the mapping which is the result when mapping manually.

The main goal is to not have to repeat the mapping behavior everytime it happens to be nested.

[boppercr]

Does the code that I gave is sufficient to understand the issue or do you require more information?

[roji]

@boppercr I unfortunately haven't gotten around to looking at this issue - with the .NET 9 release this week a lot of time went into that... As things stabilize we'll have more time to perform investigations - feel free to ping me again in a few weeks' time if there's still no answer, and thanks for your patience!

[roji]

In your lambda:

public static Expression<Func<ClassProgram, ProgramDTO>> ToProgramDTO => program => new ProgramDTO  
{  
    Id = program.Id,  
    Session = SessionMapper.ToSessionDTO.Compile()(program.Session)  
    //Posts = blog.Posts.Select(PostMapper.ToPostDTO).ToList()  
};

Compile() compiles the ToSessionDTO expression tree, returning a Func<Session,SessionID> - this is not an expression tree, but runnable .NET code, just as if you called a regular .NET function there. Since it's not an expression tree, EF cannot see it and translate it to SQL, so it can't add the JOIN you're expecting. What's happening is that EF simply invokes your function, since ToProgramDTO is passed to the top-level select, so since the lambda isn't fully translatable, that part of it (the result of Compile()) is evaluated client-side, in .NET, and does not affect the SQL.

Composing/nesting expression trees can indeed be tricky. You can do this by doing manual expression tree construction via the Expression APIs (e.g. Expression.Lambda), at which point you can create any expression tree structure you want, including nesting. But that is quite an advanced API, and very easy to get wrong - I'd recommend staying away from it unless you really need to. I understand you're trying to nest expression trees as a way of reusing code, but in this instance you're probably better off simply doing a bit of repetition (i.e. in the way that your ToExpectedProgramDTO looks like).

I'll go ahead and close this as everything is working as expected, but if you have further questions or need more guidance, feel free to post back here.

[boppercr]

Thanks for the answer. The Expression API is indeed tricky and that's a reason why we wanted to have a simpler approach. I also noticed that using a constructor that receives the actual entity to perform the mapping isn't translated as well, but it probably the same issue you mentionned where it's client-side and not translatable to SQL?

[roji]

I'm not sure exactly to what you're referring to (a code sample is always best when asking a question), but yeah, that's probable.

[boppercr]

I adjusted the project to reflect what I was talking about. efcore-issue-34866-2.zip

[roji]

Yes, see #32812.

[boppercr]

So, with EF 9.0, it'll handle the constructors recursively and therefor the scenario of the second project would be handled properly? Or did I understand that totally wrong?