suppose the former attribute declaration is <code>field :position</code>, after deployed in production for a while, there're many documents with empty position,
so I've to changed to <code>field :position, :default => 'manager'</code>, but the default value only works for new records, it could be terrific to overload the default value when loading the existed documents.

currently my ugly crack is:

field :position, :default => 'manager'
def position;
self[:position] || 'manager'
end