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

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?

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).

If you are using a DbContext, you can query the .Local collection, and the == operator will work also with objects:

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

        db.Lecturers.Load() //this is optional, it may take some time in the first load

        //Add .Local to this line
        var toRemove = db.Lecturers.Local 
                        .Where(l => l.Courses.Contains(courseFromDb)).ToList();

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

        db.SaveChanges();
    }
}

The .Local is an ObservableCollection, so you can compare anything you like inside it (not limited to SQL queries which don't support object comparison). Just to make sure you get all your objects in the .Local collection you can call the db.Lecturers.Load() method before calling .Local, which brings all database entries into the Local collection.

The Courses collection of below line should be null or empty.

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