Fixed #29547 -- Added support for partial indexes.

Thanks to Ian Foote, Mariusz Felisiak, Simon Charettes, and
Markus Holtermann for comments and feedback.

tests/model_indexes/tests.py

@@ -1,5 +1,6 @@
 from django.conf import settings
 from django.db import connection, models
+from django.db.models.query_utils import Q
 from django.test import SimpleTestCase, skipUnlessDBFeature
 from django.test.utils import isolate_apps
 
@@ -14,8 +15,10 @@ class IndexesTests(SimpleTestCase):
     def test_repr(self):
         index = models.Index(fields=['title'])
         multi_col_index = models.Index(fields=['title', 'author'])
+        partial_index = models.Index(fields=['title'], name='long_books_idx', condition=Q(pages__gt=400))
         self.assertEqual(repr(index), "<Index: fields='title'>")
         self.assertEqual(repr(multi_col_index), "<Index: fields='title, author'>")
+        self.assertEqual(repr(partial_index), "<Index: fields='title', condition=(AND: ('pages__gt', 400))>")
 
     def test_eq(self):
         index = models.Index(fields=['title'])
@@ -52,6 +55,14 @@ class IndexesTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, msg):
             models.Index(name='test_opclass', fields=['field', 'other'], opclasses=['jsonb_path_ops'])
 
+    def test_condition_requires_index_name(self):
+        with self.assertRaisesMessage(ValueError, 'An index must be named to use condition.'):
+            models.Index(condition=Q(pages__gt=400))
+
+    def test_condition_must_be_q(self):
+        with self.assertRaisesMessage(ValueError, 'Index.condition must be a Q instance.'):
+            models.Index(condition='invalid', name='long_book_idx')
+
     def test_max_name_length(self):
         msg = 'Index names cannot be longer than 30 characters.'
         with self.assertRaisesMessage(ValueError, msg):
@@ -110,6 +121,25 @@ class IndexesTests(SimpleTestCase):
             {'fields': ['title'], 'name': 'model_index_title_196f42_idx', 'db_tablespace': 'idx_tbls'}
         )
 
+    def test_deconstruct_with_condition(self):
+        index = models.Index(
+            name='big_book_index',
+            fields=['title'],
+            condition=Q(pages__gt=400),
+        )
+        index.set_name_with_model(Book)
+        path, args, kwargs = index.deconstruct()
+        self.assertEqual(path, 'django.db.models.Index')
+        self.assertEqual(args, ())
+        self.assertEqual(
+            kwargs,
+            {
+                'fields': ['title'],
+                'name': 'model_index_title_196f42_idx',
+                'condition': Q(pages__gt=400),
+            }
+        )
+
     def test_clone(self):
         index = models.Index(fields=['title'])
         new_index = index.clone()