Error UNIQUE constraint failed with Django 1.7

I got error (Error UNIQUE constraint failed) when tried create an object with create function. Many websites post the solution is update sequence of database, but the problem isn't solved. I had this code:

TutorUser.objects.create(
            user=user,
            birthdate=date_obj,
            name=request.POST['name'],
            usertype="tutor")

I solved the problem, changing to this:

tu = TutorUser(
            user=user,
            birthdate=date_obj,
            name=request.POST['name'],
            usertype="tutor")
tu.save()

The question is ... ¿is that a bug of Django 1.7?

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