Entity framework performing an Insert, when it should be doing an Update

I am having a real issue with the EF v1. I have quite a big EDMX with maybe 50 entities mapped, but this one entity is causing me grief.

The entity has mappings to other entities which in effect are reference tables, but for some reason it is trying to do an insert and not just update itself.

Here is a fragment of my code:

using (var context = new someEntities()) {
    var studentCourseJoin =
        context.StudentCourseJoinSet.Where(o => o.Code == scjCode).First();

    studentCourseJoin.EntryStatus = new EntryStatus { Code = viewModel.StudentDetails.EntryStatusCode };
    studentCourseJoin.ParentalInHigherEducation = new ParentalInHigherEducation { Code = viewModel.StudentDetails.ParentalInHigherEducationCode }开发者_Python百科;
    studentCourseJoin.School = new School { Code = viewModel.StudentDetails.SchoolCode };

    studentCourseJoin.Institution = new Institution { Code = viewModel.StudentDetails.InstitutionCode };

    studentCourseJoin.LastSchoolEndYear = viewModel.StudentDetails.LastSchoolEndYear;
    studentCourseJoin.LastInstitutionEndYear = viewModel.StudentDetails.LastInstitutionEndYear;

    // Blows up here trying to do an insert on the studentCourseJoin.Institution.
    // But if I removed this one, then it will blow up on another one.
    context.SaveChanges(true);
}

If anyone has ANY ideas please, they would help a lot.

Try adding those lines before calling SaveChanges:

ObjectStateEntry entry = context.ObjectStateManager.GetObjectStateEntry(studentCourseJoin);
entry.ChangeState(EntityState.Modified);

Update:

Try this for Institution instead:

studentCourseJoin.Institution = context.Institutions.FirstOrDefault(i => i.Code == viewModel.StudentDetails.InstitutionCode);