为carrierwave设置动态上传器

Question

我希望根据对象属性动态设置上传器。

#Something like this in the user.rb
mount_uploader :avatar, Avatar2Uploader,:if => Proc.new{|p| p.migrated == false }
mount_uploader :avatar, Avatar1Uploader,:if => Proc.new{|p| p.migrated == true }

有没有办法实现这个目标？