IQueryable not generating the desired query in entity framework core - api

Question

I have some entities similar to the following:
public class Teacher
{
public int TeacherId { get; set; }
public string TeacherName { get; set; }
public int Age { get; set; }
public string CurrentGrade { get; set; }
public int YearsTeaching { get; set; }
pubilc ICollection<StudentFeedback> StudentFeedback { get; set; }
}
public class StudentFeedback
{
public int StudentFeedBackId { get; set; }
public int TeacherId { get; set; }
public int StudentId { get; set; }
public string Feedback { get; set; }
public Teacher Teacher { get; set; }
public Student Student { get; set; }
}
public class Student
{
public int StudentId { get; set; }
public string StudentName { get; set; }
public int Age { get; set; }
public string CurrentGrade { get; set; }
}
I have a repository with a method where I want to return a teacher or list of teachers where the StudentFeedback returned belongs to the student who is looking at it (the studentId is stored in a token).
So, lets say I have the Teacher (teacherId) and a Student (userId) who is hitting the API endpoint. I currently have the following:
int teacherId = 2;
int userId = 20; // This is the currently logged in user, extracted from the token.
var query = _context.Teachers.AsQueryable();/* _context is the DataContext*/
query = query.Where(t => p.TeacherId == teacherId);
query = query.Where(u => u.StudentFeedback.Any(x => x.StudentId == userId));
However this is still returning all StudentFeedback from all Students, so long as the userId (student) has feedback provided for the teacher in question. I had a look at the query that gets executed and the problem is that the studentId predicate is in the wrong place. A very rough version of the query is:
SELECT *
FROM ( SELECT t.*
FROM dbo.Teachers t
WHERE (t.TeacherId = 2)
AND EXISTS ( SELECT 1
FROM dbo.StudentFeedback t0
WHERE (t.TeacherId = t0.TeacherId)
AND (t0.StudentId = 20))) p
LEFT JOIN dbo.StudentFeedback sf ON p.TeacherId = sf.TeacherId
Whereas it should be something like
SELECT *
FROM ( SELECT t.*
FROM dbo.Teachers t
WHERE (t.TeacherId = 2)) p
LEFT JOIN dbo.StudentFeedback sf ON p.TeacherId = sf.TeacherId
AND sf.StudentId = 20
but I don't know how to make that happen. Is there something wrong with the IQueryable predicates I've setup or have I missed some logic in the modelBuilder within the datacontext?
Thank-you.
Edit: I am using Entity Framework Core 5.0.2 and I am also using Automapper with the following code:
query.ProjectTo<TeacherDTO>(_mapper.ConfigurationProvider).AsNoTracking()
Here is what I am getting back currently:
[
{
"teacherid": 2,
"teacherName": "Jane Smith",
"age": 35,
"currentGrade": "One",
"yearsTeaching": 12,
"studentFeedback": [
{
"studentFeedBackId": 12,
"teacherId": 6,
"studentId": 20,
"feedback": "Ms Smith is my favorite teacher"
} ,
{
"studentFeedBackId": 16,
"teacherId": 6,
"studentId": 43,
"feedback": "Ms Smith was so kind to me"
} ,
{
"studentFeedBackId": 21,
"teacherId": 6,
"studentId": 89,
"feedback": "Thank you Mrs Smith for being my teacher. I learned a lot."
}
]
}
]
here is what I want to be getting back:
[
{
"teacherid": 2,
"teacherName": "Jane Smith",
"age": 35,
"currentGrade": "One",
"yearsTeaching": 12,
"studentFeedback": [
{
"studentFeedBackId": 12,
"teacherId": 6,
"studentId": 20,
"feedback": "Ms Smith is my favorite teacher"
}
]
}
]

Answer 1

Thanks #LucianBargaoanu for pointing me in the right direction by saying to have the where in the mapping itself. The solution is to use Parameterization when using Automapper:
The code from this pages shows an example:
string currentUserName = null;
cfg.CreateMap<Course, CourseModel>()
.ForMember(m => m.CurrentUserName, opt => opt.MapFrom(src => currentUserName));
and then
dbContext.Courses.ProjectTo<CourseModel>(Config, new { currentUserName = Request.User.Name });

Answer 2

If you want a join you should use Join method in your linq statement. See https://www.tutorialsteacher.com/linq/linq-joining-operator-join . You got exactly what you wrote in your query. Where(u => u.StudentFeedback.Any(x => x.StudentId == userId));
.Any translates to exists.

Answer 3

If you use Net5 EF you can just add a Students property to the Teacher class:
public class Teacher
{
.....
pubilc ICollection<Student> Students { get; set; }
pubilc ICollection<StudentFeedback> StudentFeedbacks { get; set; }
}
You can use query this way:
var query = _context.Teachers.Include(i=> i.StudentFeedbacks)
.Where(t =>
t.TeacherId == teacherId
&& t.StudentFeedbacks.Any(x => x.StudentId == userId))
.ToArray();