Reorganized text db function tests.

tests/db_functions/text/test_left.py

@@ -0,0 +1,27 @@
+from django.db.models import CharField, 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)
+        # If alias is null, set it to the first 2 lower characters of the name.
+        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=CharField()))
+        self.assertQuerysetEqual(authors.order_by('name'), ['Joh', 'Rho'], lambda a: a.name_part)