Can't convert 'DeferredAttribute' object to str implicitly - django

Question

When Iam trying to get the degree of a student and do the return value with that code:
def __str__(self):
return self.student_subject + " " + Student.last_name
I got the above error message.
Here is the whole Django code:
from django.db import models
# Create your models here.
class Student(models.Model):
first_name = models.CharField(max_length=15)
last_name = models.CharField(max_length=15)
age = models.IntegerField(default=0)
birth_date = models.DateTimeField()
def __str__(self):
return self.first_name + " " + self.last_name
class Degree(models.Model):
student_id = models.ForeignKey(Student, on_delete=models.CASCADE)
student_subject = models.CharField(max_length=150)
student_degree = models.IntegerField(default=0)
def __str__(self):
return self.student_subject + " " + Student.last_name
the error is happening because I trying to inherite the return from the above class Student
And here is the error message:
TypeError at /admin/sellingportal/degree/
Can't convert 'DeferredAttribute' object to str implicitly
Request Method: GET
Request URL: http://127.0.0.1:8000/admin/sellingportal/degree/
Django Version: 1.10.1
Exception Type: TypeError
Exception Value:
Can't convert 'DeferredAttribute' object to str implicitly
Exception Location: C:\Users\Muham\PycharmProjects\managementsite\managementstudio\sellingportal\models.py in __str__, line 22
Python Executable: C:\Python34\python.exe
Python Version: 3.4.4
Python Path:
['C:\\Users\\Muham\\PycharmProjects\\managementsite\\managementstudio',
'C:\\Windows\\SYSTEM32\\python34.zip',
'C:\\Python34\\DLLs',
'C:\\Python34\\lib',
'C:\\Python34',
'C:\\Python34\\lib\\site-packages']
Server time: Fri, 16 Sep 2016 01:04:08 +0200

Answer 1

Just change it to:
def __str__(self):
return self.student_subject + " " + self.student_id.last_name
and all should be fine.
You cannot set Student.last_name because you have to show from which object, should the last name be taken from.