Show user posts by area

Still don't want to mention `current` anywhere in a model (at least not in the view sense, which this implies). Try:

``` ruby
def posts_for_area(area)
  area.posts.where(
    "user_id = :user_id and area_id = :area_id",
     user_id: self.id, area_id: area.id
  )
end
```