[1.9.x] Fixed #26034 -- Fixed incorrect index handling on PostgreSQL on Char/TextField with unique=True and db_index=True.

Thanks Simon Charette for review.

Backport of 56aaae58a746eb39d5e92ba60f59f4c750a8e1a8 from master

4 files changed, 72 insertions, 2 deletions

diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index cc31aadf0a..8d4dc871cd 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -110,13 +110,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             new_db_params, strict,
         )
         # Added an index? Create any PostgreSQL-specific indexes.
-        if ((not old_field.db_index and new_field.db_index) or (not old_field.unique and new_field.unique)):
+        if not old_field.db_index and not old_field.unique and (new_field.db_index or new_field.unique):
             like_index_statement = self._create_like_index_sql(model, new_field)
             if like_index_statement is not None:
                 self.execute(like_index_statement)
 
         # Removed an index? Drop any PostgreSQL-specific indexes.
-        if ((not new_field.db_index and old_field.db_index) or (not new_field.unique and old_field.unique)):
+        if (old_field.db_index or old_field.unique) and not (new_field.db_index or new_field.unique):
             index_to_remove = self._create_index_name(model, [old_field.column], suffix='_like')
             index_names = self._constraint_names(model, [old_field.column], index=True)
             for index_name in index_names:
diff --git a/docs/releases/1.8.9.txt b/docs/releases/1.8.9.txt
index 5525eca84a..080d6506e8 100644
--- a/docs/releases/1.8.9.txt
+++ b/docs/releases/1.8.9.txt
@@ -18,3 +18,8 @@ Bugfixes
 * Fixed a regression that caused the incorrect day to be selected when opening
   the admin calendar widget for timezones from GMT+0100 to GMT+1200
   (:ticket:`24980`).
+
+* Fixed incorrect index handling in migrations on PostgreSQL when adding
+  ``db_index=True`` or ``unique=True`` to a ``CharField`` or ``TextField`` that
+  already had the other specified, or when removing one of them from a field
+  that had both (:ticket:`26034`).
diff --git a/docs/releases/1.9.2.txt b/docs/releases/1.9.2.txt
index ffe5030d9e..d7857052ad 100644
--- a/docs/releases/1.9.2.txt
+++ b/docs/releases/1.9.2.txt
@@ -25,3 +25,8 @@ Bugfixes
 * Fixed a regression in the admin's edit related model popup that caused an
   escaped value to be displayed in the select dropdown of the parent window
   (:ticket:`25997`).
+
+* Fixed incorrect index handling in migrations on PostgreSQL when adding
+  ``db_index=True`` or ``unique=True`` to a ``CharField`` or ``TextField`` that
+  already had the other specified, or when removing one of them from a field
+  that had both (:ticket:`26034`).
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 7e17402f70..dfd0659d34 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -1757,3 +1757,63 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(Note, new_field, old_field, strict=True)
         self.assertEqual(self.get_constraints_for_column(Note, 'info'), [])
+
+    @unittest.skipUnless(connection.vendor == 'postgresql', "PostgreSQL specific")
+    def test_alter_field_add_unique_to_charfield_with_db_index(self):
+        # Create the table and verify initial indexes.
+        with connection.schema_editor() as editor:
+            editor.create_model(BookWithoutAuthor)
+        self.assertEqual(
+            self.get_constraints_for_column(BookWithoutAuthor, 'title'),
+            ['schema_book_d5d3db17', 'schema_book_title_2dfb2dff_like']
+        )
+        # Alter to add unique=True (should add 1 index)
+        old_field = BookWithoutAuthor._meta.get_field('title')
+        new_field = CharField(max_length=100, db_index=True, unique=True)
+        new_field.set_attributes_from_name('title')
+        with connection.schema_editor() as editor:
+            editor.alter_field(BookWithoutAuthor, old_field, new_field, strict=True)
+        self.assertEqual(
+            self.get_constraints_for_column(BookWithoutAuthor, 'title'),
+            ['schema_book_d5d3db17', 'schema_book_title_2dfb2dff_like', 'schema_book_title_2dfb2dff_uniq']
+        )
+        # Alter to remove unique=True (should drop unique index) # XXX: bug!
+        old_field = BookWithoutAuthor._meta.get_field('title')
+        new_field = CharField(max_length=100, db_index=True)
+        new_field.set_attributes_from_name('title')
+        with connection.schema_editor() as editor:
+            editor.alter_field(BookWithoutAuthor, old_field, new_field, strict=True)
+        self.assertEqual(
+            self.get_constraints_for_column(BookWithoutAuthor, 'title'),
+            ['schema_book_d5d3db17', 'schema_book_title_2dfb2dff_like', 'schema_book_title_2dfb2dff_uniq']
+        )
+
+    @unittest.skipUnless(connection.vendor == 'postgresql', "PostgreSQL specific")
+    def test_alter_field_add_db_index_to_charfield_with_unique(self):
+        # Create the table and verify initial indexes.
+        with connection.schema_editor() as editor:
+            editor.create_model(Tag)
+        self.assertEqual(
+            self.get_constraints_for_column(Tag, 'slug'),
+            ['schema_tag_slug_2c418ba3_like', 'schema_tag_slug_key']
+        )
+        # Alter to add db_index=True
+        old_field = Tag._meta.get_field('slug')
+        new_field = SlugField(db_index=True, unique=True)
+        new_field.set_attributes_from_name('slug')
+        with connection.schema_editor() as editor:
+            editor.alter_field(Tag, old_field, new_field, strict=True)
+        self.assertEqual(
+            self.get_constraints_for_column(Tag, 'slug'),
+            ['schema_tag_slug_2c418ba3_like', 'schema_tag_slug_key']
+        )
+        # Alter to remove db_index=True
+        old_field = Tag._meta.get_field('slug')
+        new_field = SlugField(unique=True)
+        new_field.set_attributes_from_name('slug')
+        with connection.schema_editor() as editor:
+            editor.alter_field(Tag, old_field, new_field, strict=True)
+        self.assertEqual(
+            self.get_constraints_for_column(Tag, 'slug'),
+            ['schema_tag_slug_2c418ba3_like', 'schema_tag_slug_key']
+        )