Refs #28643 -- Added math database functions.

Thanks Nick Pope for much review.

tests/db_functions/math/test_mod.py

@@ -0,0 +1,36 @@
+import math
+from decimal import Decimal
+
+from django.db.models.functions import Mod
+from django.test import TestCase
+
+from ..models import DecimalModel, FloatModel, IntegerModel
+
+
+class ModTests(TestCase):
+
+    def test_decimal(self):
+        DecimalModel.objects.create(n1=Decimal('-9.9'), n2=Decimal('4.6'))
+        obj = DecimalModel.objects.annotate(n_mod=Mod('n1', 'n2')).first()
+        self.assertIsInstance(obj.n_mod, Decimal)
+        self.assertAlmostEqual(obj.n_mod, Decimal(math.fmod(obj.n1, obj.n2)))
+
+    def test_float(self):
+        FloatModel.objects.create(f1=-25, f2=0.33)
+        obj = FloatModel.objects.annotate(f_mod=Mod('f1', 'f2')).first()
+        self.assertIsInstance(obj.f_mod, float)
+        self.assertAlmostEqual(obj.f_mod, math.fmod(obj.f1, obj.f2))
+
+    def test_integer(self):
+        IntegerModel.objects.create(small=20, normal=15, big=1)
+        obj = IntegerModel.objects.annotate(
+            small_mod=Mod('small', 'normal'),
+            normal_mod=Mod('normal', 'big'),
+            big_mod=Mod('big', 'small'),
+        ).first()
+        self.assertIsInstance(obj.small_mod, float)
+        self.assertIsInstance(obj.normal_mod, float)
+        self.assertIsInstance(obj.big_mod, float)
+        self.assertEqual(obj.small_mod, math.fmod(obj.small, obj.normal))
+        self.assertEqual(obj.normal_mod, math.fmod(obj.normal, obj.big))
+        self.assertEqual(obj.big_mod, math.fmod(obj.big, obj.small))