SoftDeleteMixin: allow for None values

This is a fixup for I13c6233cfeb611b1b106eedfc9b57d2af313c46b to
address the remaining comments:

  - allow for None values to be passed (as column deleted is nullable)

  - document why column deleted is not of type boolean

Change-Id: I04f690f7a1bb8cb14f1e3322643a8cf8bc9c549a

oslo_db/sqlalchemy/types.py

@@ -84,9 +84,20 @@ class SoftDeleteInteger(TypeDecorator):
     layer by the means of a custom SQLAlchemy type decorator makes sure we
     always pass a proper integer value to a DBAPI implementation.
 
+    This is not a general purpose boolean integer type as it specifically
+    allows for arbitrary positive integers outside of the boolean int range
+    (0, 1, False, True), so that it's possible to have compound unique
+    constraints over multiple columns including ``deleted`` (e.g. to
+    soft-delete flavors with the same name in Nova without triggering
+    a constraint violation): ``deleted`` is set to be equal to a PK
+    int value on deletion, 0 denotes a non-deleted row.
+
     """
 
     impl = Integer
 
     def process_bind_param(self, value, dialect):
-        return int(value)
+        if value is None:
+            return None
+        else:
+            return int(value)

oslo_db/tests/sqlalchemy/test_models.py

@@ -229,3 +229,13 @@ class SoftDeleteMixinTest(test_base.DbTestCase):
         m = SoftDeletedModel(id=1, smth='test', deleted=False)
         self.session.add(m)
         self.session.commit()
+
+    def test_deleted_set_to_null(self):
+        m = SoftDeletedModel(id=123456, smth='test')
+        self.session.add(m)
+        self.session.commit()
+
+        m.deleted = None
+        self.session.commit()
+
+        self.assertIsNone(m.deleted)