Fixed #26709 -- Added class-based indexes.

Added the AddIndex and RemoveIndex operations to use them in migrations.

Thanks markush, mjtamlyn, timgraham, and charettes for review and advice.

tests/model_indexes/tests.py

@@ -0,0 +1,62 @@
+from django.db import models
+from django.test import TestCase
+
+from .models import Book
+
+
+class IndexesTests(TestCase):
+
+    def test_repr(self):
+        index = models.Index(fields=['title'])
+        multi_col_index = models.Index(fields=['title', 'author'])
+        self.assertEqual(repr(index), "<Index: fields='title'>")
+        self.assertEqual(repr(multi_col_index), "<Index: fields='title, author'>")
+
+    def test_eq(self):
+        index = models.Index(fields=['title'])
+        same_index = models.Index(fields=['title'])
+        another_index = models.Index(fields=['title', 'author'])
+        self.assertEqual(index, same_index)
+        self.assertNotEqual(index, another_index)
+
+    def test_raises_error_without_field(self):
+        msg = 'At least one field is required to define an index.'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.Index()
+
+    def test_max_name_length(self):
+        msg = 'Index names cannot be longer than 30 characters.'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.Index(fields=['title'], name='looooooooooooong_index_name_idx')
+
+    def test_name_constraints(self):
+        msg = 'Index names cannot start with an underscore (_).'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.Index(fields=['title'], name='_name_starting_with_underscore')
+
+        msg = 'Index names cannot start with a number (0-9).'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.Index(fields=['title'], name='5name_starting_with_number')
+
+    def test_name_auto_generation(self):
+        index = models.Index(fields=['author'])
+        index.model = Book
+        self.assertEqual(index.name, 'model_index_author_0f5565_idx')
+
+        # fields may be truncated in the name. db_column is used for naming.
+        long_field_index = models.Index(fields=['pages'])
+        long_field_index.model = Book
+        self.assertEqual(long_field_index.name, 'model_index_page_co_69235a_idx')
+
+        # suffix can't be longer than 3 characters.
+        long_field_index.suffix = 'suff'
+        msg = 'Index too long for multiple database support. Is self.suffix longer than 3 characters?'
+        with self.assertRaisesMessage(AssertionError, msg):
+            long_field_index.get_name()
+
+    def test_deconstruction(self):
+        index = models.Index(fields=['title'])
+        path, args, kwargs = index.deconstruct()
+        self.assertEqual(path, 'django.db.models.Index')
+        self.assertEqual(args, ())
+        self.assertEqual(kwargs, {'fields': ['title']})