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.

django/db/backends/base/schema.py

@@ -3,6 +3,7 @@ import logging
 from datetime import datetime
 
 from django.db.backends.utils import strip_quotes
+from django.db.models import Index
 from django.db.transaction import TransactionManagementError, atomic
 from django.utils import timezone
 from django.utils.encoding import force_bytes
@@ -540,8 +541,16 @@ class BaseDatabaseSchemaEditor:
         # True               | False            | True               | True
         if old_field.db_index and not old_field.unique and (not new_field.db_index or new_field.unique):
             # Find the index for this field
-            index_names = self._constraint_names(model, [old_field.column], index=True)
+            meta_index_names = {index.name for index in model._meta.indexes}
+            # Retrieve only BTREE indexes since this is what's created with
+            # db_index=True.
+            index_names = self._constraint_names(model, [old_field.column], index=True, type_=Index.suffix)
             for index_name in index_names:
+                if index_name in meta_index_names:
+                    # There only way to check if an index was created with
+                    # db_index=True or with Index(['field'], name='foo')
+                    # is to look at its name (refs #28053).
+                    continue
                 self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
         # Change check constraints?
         if old_db_params['check'] != new_db_params['check'] and old_db_params['check']:
@@ -942,7 +951,7 @@ class BaseDatabaseSchemaEditor:
 
     def _constraint_names(self, model, column_names=None, unique=None,
                           primary_key=None, index=None, foreign_key=None,
-                          check=None):
+                          check=None, type_=None):
         """Return all constraint names matching the columns and conditions."""
         if column_names is not None:
             column_names = [
@@ -964,5 +973,7 @@ class BaseDatabaseSchemaEditor:
                     continue
                 if foreign_key is not None and not infodict['foreign_key']:
                     continue
+                if type_ is not None and infodict['type'] != type_:
+                    continue
                 result.append(name)
         return result

docs/releases/1.11.1.txt

@@ -72,3 +72,6 @@ Bugfixes
 
 * Prevented ``AddIndex`` and ``RemoveIndex`` from mutating model state
   (:ticket:`28043`).
+
+* Prevented migrations from dropping database indexes from ``Meta.indexes``
+  when changing ``Field.db_index`` to ``False`` (:ticket:`28052`).

tests/schema/models.py

@@ -33,6 +33,13 @@ class AuthorWithEvenLongerName(models.Model):
         apps = new_apps
 
 
+class AuthorWithIndexedName(models.Model):
+    name = models.CharField(max_length=255, db_index=True)
+
+    class Meta:
+        apps = new_apps
+
+
 class Book(models.Model):
     author = models.ForeignKey(Author, models.CASCADE)
     title = models.CharField(max_length=100, db_index=True)

tests/schema/tests.py

@@ -29,10 +29,11 @@ from .fields import (
     CustomManyToManyField, InheritedManyToManyField, MediumBlobField,
 )
 from .models import (
-    Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName, Book,
+    Author, AuthorWithDefaultHeight, AuthorWithEvenLongerName,
-    BookForeignObj, BookWeak, BookWithLongName, BookWithO2O, BookWithoutAuthor,
+    AuthorWithIndexedName, Book, BookForeignObj, BookWeak, BookWithLongName,
-    BookWithSlug, IntegerPK, Node, Note, NoteRename, Tag, TagIndexed,
+    BookWithO2O, BookWithoutAuthor, BookWithSlug, IntegerPK, Node, Note,
-    TagM2MTest, TagUniqueRename, Thing, UniqueTest, new_apps,
+    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