From 02365d3f38a64a5c2f3e932f23925a381d5bb151 Mon Sep 17 00:00:00 2001
From: Tim Martin <tim@asymptotic.co.uk>
Date: Mon, 18 Sep 2017 20:54:48 +0100
Subject: [PATCH] Fixed #28542 -- Fixed deletion of primary key constraint if
 the new field is unique.

---
 django/db/backends/base/schema.py |  9 +++--
 tests/schema/tests.py             | 56 ++++++++++++++++++++-----------
 2 files changed, 40 insertions(+), 25 deletions(-)

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 4fb6174437..f1e8e66479 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -527,7 +527,7 @@ class BaseDatabaseSchemaEditor:
         # Has unique been removed?
         if old_field.unique and (not new_field.unique or (not old_field.primary_key and new_field.primary_key)):
             # Find the unique constraint for this field
-            constraint_names = self._constraint_names(model, [old_field.column], unique=True)
+            constraint_names = self._constraint_names(model, [old_field.column], unique=True, primary_key=False)
             if strict and len(constraint_names) != 1:
                 raise ValueError("Found wrong number (%s) of unique constraints for %s.%s" % (
                     len(constraint_names),
@@ -671,6 +671,9 @@ class BaseDatabaseSchemaEditor:
         if post_actions:
             for sql, params in post_actions:
                 self.execute(sql, params)
+        # If primary_key changed to False, delete the primary key constraint.
+        if old_field.primary_key and not new_field.primary_key:
+            self._delete_primary_key(model, strict)
         # Added a unique?
         if (not old_field.unique and new_field.unique) or (
             old_field.primary_key and not new_field.primary_key and new_field.unique
@@ -693,11 +696,7 @@ class BaseDatabaseSchemaEditor:
         if old_field.primary_key and new_field.primary_key and old_type != new_type:
             rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
         # Changed to become primary key?
-        # Note that we don't detect unsetting of a PK, as we assume another field
-        # will always come along and replace it.
         if not old_field.primary_key and new_field.primary_key:
-            # First, drop the old PK
-            self._delete_primary_key(model, strict)
             # Make the new one
             self.execute(
                 self.sql_create_pk % {
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index cc2d5384f8..3a66c2ae7a 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -1098,6 +1098,42 @@ class SchemaTests(TransactionTestCase):
         Should be able to rename an IntegerField(primary_key=True) to
         IntegerField(unique=True).
         """
+        with connection.schema_editor() as editor:
+            editor.create_model(IntegerPK)
+        # Delete the old PK
+        old_field = IntegerPK._meta.get_field('i')
+        new_field = IntegerField(unique=True)
+        new_field.model = IntegerPK
+        new_field.set_attributes_from_name('i')
+        with connection.schema_editor() as editor:
+            editor.alter_field(IntegerPK, old_field, new_field, strict=True)
+        # The primary key constraint is gone. Result depends on database:
+        # 'id' for SQLite, None for others (must not be 'i').
+        self.assertIn(self.get_primary_key(IntegerPK._meta.db_table), ('id', None))
+
+        # Set up a model class as it currently stands. The original IntegerPK
+        # class is now out of date and some backends make use of the whole
+        # model class when modifying a field (such as sqlite3 when remaking a
+        # table) so an outdated model class leads to incorrect results.
+        class Transitional(Model):
+            i = IntegerField(unique=True)
+            j = IntegerField(unique=True)
+
+            class Meta:
+                app_label = 'schema'
+                apps = new_apps
+                db_table = 'INTEGERPK'
+
+        # model requires a new PK
+        old_field = Transitional._meta.get_field('j')
+        new_field = IntegerField(primary_key=True)
+        new_field.model = Transitional
+        new_field.set_attributes_from_name('j')
+
+        with connection.schema_editor() as editor:
+            editor.alter_field(Transitional, old_field, new_field, strict=True)
+
+        # Create a model class representing the updated model.
         class IntegerUnique(Model):
             i = IntegerField(unique=True)
             j = IntegerField(primary_key=True)
@@ -1107,26 +1143,6 @@ class SchemaTests(TransactionTestCase):
                 apps = new_apps
                 db_table = 'INTEGERPK'
 
-        with connection.schema_editor() as editor:
-            editor.create_model(IntegerPK)
-
-        # model requires a new PK
-        old_field = IntegerPK._meta.get_field('j')
-        new_field = IntegerField(primary_key=True)
-        new_field.model = IntegerPK
-        new_field.set_attributes_from_name('j')
-
-        with connection.schema_editor() as editor:
-            editor.alter_field(IntegerPK, old_field, new_field, strict=True)
-
-        old_field = IntegerPK._meta.get_field('i')
-        new_field = IntegerField(unique=True)
-        new_field.model = IntegerPK
-        new_field.set_attributes_from_name('i')
-
-        with connection.schema_editor() as editor:
-            editor.alter_field(IntegerPK, old_field, new_field, strict=True)
-
         # Ensure unique constraint works.
         IntegerUnique.objects.create(i=1, j=1)
         with self.assertRaises(IntegrityError):