Entity Framework - Insert Related Entities

Question

I'm trying to insert an entity (document) with the EF and ASP.NET. My entity model and schema is below, and I'm getting an unusual error about a foreign key mismatch on the Tutors table.

Entities in 'SchoolEntities.Courses' participate in the 'FK_Courses_Tutors' relationship. 0 related 'Tutors' were found. 1 'Tutors' is expected.

I'm re-attaching the entities in different methods, which works if there's either:, A Student ID, no parent ID and a StudentID with a parent ID. Once I add the course it fails to work. FK_Courses_Tutors is set to cascade on both.

public static void EnrolStudent(Guid studentId, Course course) {
    Document doc = new Document() {
        Student = new Student() { StudentID = studentId },
        Course = new Course() { CourseID = course.CourseID },
        OriginalFilename = course.Name,
        IsDirectory = true
    };
    Document parentDir = FileBLL.GetDocumentByNestingLevel(doc, 1); // returns an int which is the ID of the parent directory
    doc.ParentID = parentDir.DocumentID;
    FileBLL.AddFileEntry(doc);
}

To add an entry, I'm using:

public static int AddFileEntry(Document document) {
using(var context = new SchoolEntities()) {
    if(document.Student != null) {
        var existingStudent = context.Students.Where(s => s.StudentID == document.Student.StudentID).FirstOrDefault();
        context.AttachTo("Students", existingStudent);
        document.Student = existingStudent;
    }
    if(document.Course != null) { // works without this, but I need it
        var existingCourse = context.Courses.Include("Tutor").Where(c => c.CourseID == document.Course.CourseID).FirstOrDefault();
        context.AttachTo("Courses", existingCourse);
        document.Course = existingCourse;
    }
    context.AddToDocuments(document);
    context.SaveChanges();
    return document.DocumentID;
}
}

I'd really appreciate any help as this has been troublesome from the start.

Thanks :)