EF core ThenInclude Select says "Lambda expression used inside Include is not valid"

Question

var test = await Db.Tests.Include(test => test.Questions)
                            .ThenInclude(ques => ques.Choices.Select(
                                ch => new {
                                    ch.Id, ch.OptionName, 
                                    ch.OptionText, ch.OptionDetails, 
                                    ch.IsAnswer, ch.QuestionId
                            })).AsNoTracking()
                            .FirstOrDefaultAsync(z => z.TestId == testId);

Without Select clause it works fine. But I don't need all the Choices properties, so I try to select some of the properties by using Select clause.

It throughs this error:

System.InvalidOperationException: Lambda expression used inside Include is not valid.

Can anybody tell me what's wrong here?

Answer 1

I think you want something like:

var test = await Db.Tests
    .Where(test => test.TestId == testId)
    .Include(test => test.Questions) // Shouldn't need this
    .ThenInclude(ques => ques.Choices) // Shouldn't need this
    .SelectMany(test => test.Questions.SelectMany(ques => ques.Choices.Select(ch => new {
        ch.Id,
        ch.OptionName, 
        ch.OptionText,
        ch.OptionDetails, 
        ch.IsAnswer,
        ch.QuestionId
    }))))
    .AsNoTracking();

This will give you all the choices for a particular test.

But... since you're materializing the results as a lambda, you actually shouldn't need either Include . (EF will give warnings saying that it ignored your Include in the console anyway).