Is there a way to bypass mass assignment protection?

I have a Rails 3 app which JSON encodes objects in order to store them in a Redis key/value store.

When I retrieve the objects, I'm trying to decode the JSON and instantiate them from the data like so:

def decode(json)
  self.new(ActiveSupport::JSON.decode(json)["#{self.name.downcase}"])
end

The problem is that doing this involves mass assignment which is disallowed (for good reason I'm told!) for attributes I haven't given attr_writer ability to.

Is there a way I can bypass the mass assignment protection just for this operation only?