Note
when you got the queryset from calling ModelClass.objects.filter(condition) and refer the element using [] and save it at last, which WON’T hit the database, and it will beyond your expectation. See the below codes:
a = Blog.objects.filter(title__contains="django")
a[0].last_update_time = datetime.now()
a[0].save() # won't work as you expect
print a[0].last_update_time # this will print the old one
# but you can
b = a[0]
b.last_update_time = datetime.now()
b.save() # will work as expected