c# - LINQ, Unable to create a constant value of type XXX. Only primitive types or enumeration types are supported in this context

Question

In my application I have Lecturers and they have list of Courses they can teach and when I'm deleting a course I want to remove connection to lecturers. Here's the code:

public void RemoveCourse(int courseId)
{
    using (var db = new AcademicTimetableDbContext())
    {
        var courseFromDb = db.Courses.Find(courseId);

        var toRemove = db.Lecturers
                        .Where(l => l.Courses.Contains(courseFromDb)).ToList();

        foreach (var lecturer in toRemove)
        {
            lecturer.Courses.Remove(courseFromDb);
        }

        db.SaveChanges();
    }
}

but it doesn't work. I get

NotSupportedException: Unable to create a constant value of type Course. Only primitive types or enumeration types are supported in this context.

What am I doing wrong?

Answer 1

You can't use Contains with non-primitive values. Do

Where(l => l.Courses.Select(c => c.CourseId).Contains(courseId)

(or the Id field you use).