Django ORM select_related AttributeError

In my project im trying to use an external .py file for manage data using my django app style like this:

In my django project i create a model:

class temp_test_keywords(models.Model):
    main_id = models.ForeignKey(temp_main)
    test_id = models.ForeignKey(temp_case)
    key_id = models.ForeignKey(temp_keywords)
    variable_id = models.ForeignKey(temp_variables, null=True, blank=True)

    def __str__(self):
        return '%s -> %s' % (str(self.main_id), str(self.test_id))

Well, now in my external rst.py file i start django env like this:

import sys
import os
import django

sys.path.append('core')
os.environ['DJANGO_SETTINGS_MODULE'] = 'core.settings'
django.setup()

ok, at this point i import table and create class for do some thinks with it:

from django.db import models
from django.contrib.contenttypes.fields import 
GenericForeignKey,GenericRelation
from django.contrib.contenttypes.models import ContentType
from django.db.models import Count

from frontend.models import temp_test_keywords


class PrepareRst:
    def __init__(self,test_id,t_type,log=False):
        self.rst = self.mainprep(test_id,t_type)

    def mainprep(self,test_id,t_type): 
        return self.tc_prep(test_id)

    #TestCase rst prep method
    def tc_prep(self,test_id):
        maxpar = temp_test_keywords.objects.filter(main_id = test_id).values('key_id').annotate(total=Count('variable_id')).order_by('-total').first()
        totpar = maxpar['total']

        #Part1 list creation
        count = 0
        ltouple = ()
        l1 = ["Test Case"]
        while (count < totpar):
            l1.append("")
            count += 1
        ltouple += (l1,)
        #Query for extract keywords, values
        kv = temp_test_keywords.select_related()

but when i run an AttributeError: type object 'temp_test_keywords' has no attribute 'select_related' error raise

if i start python manage.py shell from terminal the "kv = temp_test_keywords.select_related()" command works fine, why in my .py code doesn't?

Thanks in advance