Fixed #28052 -- Prevented dropping Meta.indexes when changing db_index to False.

Thanks Marc Tamlyn for the report and Ian Foote/Tim Graham for review.

tests/schema/tests.py

@@ -29,10 +29,11 @@ from .fields import (
     CustomManyToManyField, InheritedManyToManyField, MediumBlobField,
 )
 from .models import (
-    Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName, Book,
-    BookForeignObj, BookWeak, BookWithLongName, BookWithO2O, BookWithoutAuthor,
-    BookWithSlug, IntegerPK, Node, Note, NoteRename, Tag, TagIndexed,
-    TagM2MTest, TagUniqueRename, Thing, UniqueTest, new_apps,
+    Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName,
+    AuthorWithIndexedName, Book, BookForeignObj, BookWeak, BookWithLongName,
+    BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK, Node, Note,
+    NoteRename, Tag, TagIndexed, TagM2MTest, TagUniqueRename, Thing,
+    UniqueTest, new_apps,
 )
 
 
@@ -1633,6 +1634,46 @@ class SchemaTests(TransactionTestCase):
             editor.remove_index(Author, index)
         self.assertNotIn('name', self.get_indexes(Author._meta.db_table))
 
+    def test_remove_db_index_doesnt_remove_custom_indexes(self):
+        """
+        Changing db_index to False doesn't remove indexes from Meta.indexes.
+        """
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithIndexedName)
+        # Ensure the table has its index
+        self.assertIn('name', self.get_indexes(AuthorWithIndexedName._meta.db_table))
+
+        # Add the custom index
+        index = Index(fields=['-name'], name='author_name_idx')
+        author_index_name = index.name
+        with connection.schema_editor() as editor:
+            db_index_name = editor._create_index_name(
+                model=AuthorWithIndexedName,
+                column_names=('name',),
+            )
+        if connection.features.uppercases_column_names:
+            author_index_name = author_index_name.upper()
+            db_index_name = db_index_name.upper()
+        try:
+            AuthorWithIndexedName._meta.indexes = [index]
+            with connection.schema_editor() as editor:
+                editor.add_index(AuthorWithIndexedName, index)
+            old_constraints = self.get_constraints(AuthorWithIndexedName._meta.db_table)
+            self.assertIn(author_index_name, old_constraints)
+            self.assertIn(db_index_name, old_constraints)
+            # Change name field to db_index=False
+            old_field = AuthorWithIndexedName._meta.get_field('name')
+            new_field = CharField(max_length=255)
+            new_field.set_attributes_from_name('name')
+            with connection.schema_editor() as editor:
+                editor.alter_field(AuthorWithIndexedName, old_field, new_field, strict=True)
+            new_constraints = self.get_constraints(AuthorWithIndexedName._meta.db_table)
+            self.assertNotIn(db_index_name, new_constraints)
+            # The index from Meta.indexes is still in the database.
+            self.assertIn(author_index_name, new_constraints)
+        finally:
+            AuthorWithIndexedName._meta.indexes = []
+
     def test_order_index(self):
         """
         Indexes defined with ordering (ASC/DESC) defined on column