Fix error when deleting project

When deleting a project, an error like:

  delete statement on table change_conflict expected to delete X rows

might occur.  This is because the self-referential many-to-many
relationship of change conflicts was trying to delete entries from
the secondary table twice.  Handle the case of deleting a project
better.

Change-Id: Ib919e36ede14a3a1054fcd6b95ea43ccae046d3e

gertty/db.py

@@ -200,6 +200,25 @@ class Project(object):
         self.subscribed = subscribed
         self.description = description
 
+    def delete(self):
+        # This unusual delete method is to accomodate the
+        # self-referential many-to-many relationship from
+        # change_conflict.  With the default cascade configuration,
+        # the entry from the association table is deleted regardless
+        # of whether the change being deleted is change1 or change2.
+        # However, when both changes are deleted at once (only likely
+        # to happen when the entire project is deleted), SQLAlchemy
+        # cascades through both relationships and attempts to delete
+        # the entry twice.  Since we rarely delete projects, we add a
+        # special case here to delete a project's changes one at a
+        # time to avoid this situation.
+        session = Session.object_session(self)
+        for c in self.changes:
+            session.delete(c)
+            session.flush()
+            session.expire_all()
+        session.delete(self)
+
     def createChange(self, *args, **kw):
         session = Session.object_session(self)
         args = [self] + list(args)

gertty/sync.py

@@ -236,7 +236,8 @@ class SyncProjectListTask(Task):
             local_keys = set(local.keys())
 
             for name in local_keys-remote_keys:
-                session.delete(local[name])
+                self.log.info("Deleted project %s", name)
+                local[name].delete()
 
             for name in remote_keys-local_keys:
                 p = remote[name]