Using Javascript and AJAX in the Fetch()API method: returns no values as a response

100% correct!

Your Assignments.__str__ method as defined above is:

def __str__(self):
    if self.assignment:
        if self.assignment == 'selectChanged':
            return 'lastAssignmentSelected'
        else: 
            return "err"

In this specific case, if the first if is false (if self.assignment:), this function will return None instead of a string. (The else: return "err" clause only applies to the inner if, if self.assignment == "selectChanged".)

So now at this point, your JavaScript in your page should be able to make a valid request, and get a valid response from your view. Is it doing that?