@llsouder the :flash key comes from flash session middleware https://github.com/mmcgrana/ring/blob/master/ring-core/src/ring/middleware/flash.clj#L4

>If a :flash key is set on the response by the handler, a :flash key with the same value will be set on the next request that shares the same session. This is useful for small messages that persist across redirects.

@pcj I don't use mongo myself, but what specific problems have you run into?