[1.8.x] Fixed #24893 -- Fixed lack of unique constraint when changing a field from primary_key=True to unique=True

Backport of e1e6399c2c from master

django/db/backends/base/schema.py

@@ -659,7 +659,9 @@ class BaseDatabaseSchemaEditor(object):
             for sql, params in post_actions:
                 self.execute(sql, params)
         # Added a unique?
-        if not old_field.unique and new_field.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
+        ):
             self.execute(self._create_unique_sql(model, [new_field.column]))
         # Added an index?
         if (not old_field.db_index and new_field.db_index and

docs/releases/1.8.3.txt

@@ -34,3 +34,6 @@ Bugfixes
 
 * Fixed quoting of SQL when renaming a field to ``AutoField`` in PostgreSQL
   (:ticket:`24892`).
+
+* Fixed lack of unique constraint when changing a field from
+  ``primary_key=True`` to ``unique=True`` (:ticket:`24893`).

tests/schema/models.py

@@ -82,6 +82,7 @@ class BookWithSlug(models.Model):
 
 class IntegerPK(models.Model):
     i = models.IntegerField(primary_key=True)
+    j = models.IntegerField(unique=True)
 
     class Meta:
         apps = new_apps

tests/schema/tests.py

@@ -745,6 +745,45 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(IntegerPK, old_field, new_field, strict=True)
 
+    def test_alter_int_pk_to_int_unique(self):
+        """
+        Should be able to rename an IntegerField(primary_key=True) to
+        IntegerField(unique=True).
+        """
+        class IntegerUnique(Model):
+            i = IntegerField(unique=True)
+            j = IntegerField(primary_key=True)
+
+            class Meta:
+                app_label = 'schema'
+                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):
+            IntegerUnique.objects.create(i=1, j=2)
+
     def test_rename(self):
         """
         Tests simple altering of fields