if alias is null set it to the first 2 lower characters of the name if alias is null set it to the first 2 lower characters of the name
from django.db.models import IntegerField, Value
from django.db.models.functions import Left, Lower
from django.test import TestCase

from ..models import Author


class LeftTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        Author.objects.create(name="John Smith", alias="smithj")
        Author.objects.create(name="Rhonda")

    def test_basic(self):
        authors = Author.objects.annotate(name_part=Left("name", 5))
        self.assertQuerySetEqual(
            authors.order_by("name"), ["John ", "Rhond"], lambda a: a.name_part
        )
        
        Author.objects.filter(alias__isnull=True).update(alias=Lower(Left("name", 2)))
        self.assertQuerySetEqual(
            authors.order_by("name"), ["smithj", "rh"], lambda a: a.alias
        )

    def test_invalid_length(self):
        with self.assertRaisesMessage(ValueError, "'length' must be greater than 0"):
            Author.objects.annotate(raises=Left("name", 0))

    def test_expressions(self):
        authors = Author.objects.annotate(
            name_part=Left("name", Value(3, output_field=IntegerField()))
        )
        self.assertQuerySetEqual(
            authors.order_by("name"), ["Joh", "Rho"], lambda a: a.name_part
        )