c# - How to add where clause to ThenInclude

Question

I have 3 entities:

Questionnaire.cs:

public class Questionnaire
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<Question> Questions { get; set; }
}

Question.cs:

public class Question
{
    public int Id { get; set; }
    public string Text { get; set; }
    public ICollection<Answer> Answers { get; set; }
}

and Answer.cs:

public class Answer
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public string TextAnswer { get; set; }
}

So I saved the questionnaire with answers but now i want to retrieve filtered questionnaire with questions and its answers. So i wrote linq for that, but it throws me an error, is there anything i do wrong? here is the example:

questionnaire = _context.Questionnaires.Include(qn => qn.Questions)
.ThenInclude(question => question.Answers.Where(a => a.UserId == userId))
.FirstOrDefault(qn => qn.Id == questionnaireId);

And i am getting

Message = "The property expression 'q => {from Answer a in q.Answers where Equals([a].UserId, __userId_0) select [a]}' is not valid. The expression should represent a property access: 't => t.MyProperty'.

Any ideas how to solve this problem?

Answer 1

Filtering in Include or ThenInclude is not supported. Create projection by using Select:

questionnaire = _context.Questionnaires
    .Select(n => new Questionnaire
    {
        Id = n.Id,
        Name = n.Name,
        Questions = n.Questions.Select(q => new Question
        {
           Id = q.Id,
           Text = q.Text,
           Answers = q.Where(a => a.UserId == userId).ToList()
        }).ToList()
    })
    .FirstOrDefault(qn => qn.Id == questionnaireId);

There is a github issue about this problem: https://github.com/aspnet/EntityFramework/issues/3474