So the Post model should change if the format of the activity feed changes? That sounds like tight coupling.

Is that adding a responsibility that Post shouldn't care about? I've always had a bad feeling about models that had a bunch of `for_<other_system_component>`  scopes. It feels like that is really specific to another system component that will consume it. I typically prefer to have a more generic api that can be used by other objects to mix and match for what they need to consume.