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; }
          


        

Answer 1

I think you need to have a navigation property of question in your answer because answer should have an question Id . You have this FK already you just need a nav property

Your model class look like this

public class Answer
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public string TextAnswer { get; set; }
    // added in model
    public Question Question { get; set; }
} 

and query like this

  var answers = ctx.Answers.Include(q=>q.Question).Where(a =>a.UserId=="1").ToList();

Answer 2

Filtering in Include or IncludeThen 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

Answer 3

You can have navigation properties filtered on memory:

var questionnaire= _context.Questionnaire.FirstOrDefault(qn => qn.Id == questionnaireId);

questionnaire.Answers = _context.Entry(questionnaire)
 .Collection(b => b.Answers )
 .Query()
 .Where(a => a.UserId == userId).ToList();