Fixed #27718 -- Added QuerySet.union(), intersection(), difference().

Thanks Mariusz Felisiak for review and Oracle assistance.
Thanks Tim Graham for review and writing docs.

tests/queries/test_qs_combinators.py

@@ -0,0 +1,111 @@
+from __future__ import unicode_literals
+
+from django.db.models import F, IntegerField, Value
+from django.db.utils import DatabaseError
+from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
+from django.utils.six.moves import range
+
+from .models import Number, ReservedName
+
+
+@skipUnlessDBFeature('supports_select_union')
+class QuerySetSetOperationTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        Number.objects.bulk_create(Number(num=i) for i in range(10))
+
+    def number_transform(self, value):
+        return value.num
+
+    def assertNumbersEqual(self, queryset, expected_numbers, ordered=True):
+        self.assertQuerysetEqual(queryset, expected_numbers, self.number_transform, ordered)
+
+    def test_simple_union(self):
+        qs1 = Number.objects.filter(num__lte=1)
+        qs2 = Number.objects.filter(num__gte=8)
+        qs3 = Number.objects.filter(num=5)
+        self.assertNumbersEqual(qs1.union(qs2, qs3), [0, 1, 5, 8, 9], ordered=False)
+
+    @skipUnlessDBFeature('supports_select_intersection')
+    def test_simple_intersection(self):
+        qs1 = Number.objects.filter(num__lte=5)
+        qs2 = Number.objects.filter(num__gte=5)
+        qs3 = Number.objects.filter(num__gte=4, num__lte=6)
+        self.assertNumbersEqual(qs1.intersection(qs2, qs3), [5], ordered=False)
+
+    @skipUnlessDBFeature('supports_select_difference')
+    def test_simple_difference(self):
+        qs1 = Number.objects.filter(num__lte=5)
+        qs2 = Number.objects.filter(num__lte=4)
+        self.assertNumbersEqual(qs1.difference(qs2), [5], ordered=False)
+
+    def test_union_distinct(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.all()
+        self.assertEqual(len(list(qs1.union(qs2, all=True))), 20)
+        self.assertEqual(len(list(qs1.union(qs2))), 10)
+
+    def test_union_bad_kwarg(self):
+        qs1 = Number.objects.all()
+        msg = "union() received an unexpected keyword argument 'bad'"
+        with self.assertRaisesMessage(TypeError, msg):
+            self.assertEqual(len(list(qs1.union(qs1, bad=True))), 20)
+
+    def test_limits(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.all()
+        self.assertEqual(len(list(qs1.union(qs2)[:2])), 2)
+
+    def test_ordering(self):
+        qs1 = Number.objects.filter(num__lte=1)
+        qs2 = Number.objects.filter(num__gte=2, num__lte=3)
+        self.assertNumbersEqual(qs1.union(qs2).order_by('-num'), [3, 2, 1, 0])
+
+    @skipUnlessDBFeature('supports_slicing_ordering_in_compound')
+    def test_ordering_subqueries(self):
+        qs1 = Number.objects.order_by('num')[:2]
+        qs2 = Number.objects.order_by('-num')[:2]
+        self.assertNumbersEqual(qs1.union(qs2).order_by('-num')[:4], [9, 8, 1, 0])
+
+    @skipIfDBFeature('supports_slicing_ordering_in_compound')
+    def test_unsupported_ordering_slicing_raises_db_error(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.all()
+        msg = 'LIMIT/OFFSET not allowed in subqueries of compound statements'
+        with self.assertRaisesMessage(DatabaseError, msg):
+            list(qs1.union(qs2[:10]))
+        msg = 'ORDER BY not allowed in subqueries of compound statements'
+        with self.assertRaisesMessage(DatabaseError, msg):
+            list(qs1.order_by('id').union(qs2))
+
+    @skipIfDBFeature('supports_select_intersection')
+    def test_unsupported_intersection_raises_db_error(self):
+        qs1 = Number.objects.all()
+        qs2 = Number.objects.all()
+        msg = 'intersection not supported on this database backend'
+        with self.assertRaisesMessage(DatabaseError, msg):
+            list(qs1.intersection(qs2))
+
+    def test_combining_multiple_models(self):
+        ReservedName.objects.create(name='99 little bugs', order=99)
+        qs1 = Number.objects.filter(num=1).values_list('num', flat=True)
+        qs2 = ReservedName.objects.values_list('order')
+        self.assertEqual(list(qs1.union(qs2).order_by('num')), [1, 99])
+
+    def test_order_raises_on_non_selected_column(self):
+        qs1 = Number.objects.filter().annotate(
+            annotation=Value(1, IntegerField()),
+        ).values('annotation', num2=F('num'))
+        qs2 = Number.objects.filter().values('id', 'num')
+        # Should not raise
+        list(qs1.union(qs2).order_by('annotation'))
+        list(qs1.union(qs2).order_by('num2'))
+        msg = 'ORDER BY term does not match any column in the result set'
+        # 'id' is not part of the select
+        with self.assertRaisesMessage(DatabaseError, msg):
+            list(qs1.union(qs2).order_by('id'))
+        # 'num' got realiased to num2
+        with self.assertRaisesMessage(DatabaseError, msg):
+            list(qs1.union(qs2).order_by('num'))
+        # switched order, now 'exists' again:
+        list(qs2.union(qs1).order_by('num'))