Declare once how fields map from one object to another, and Mappable compiles it into plain Ruby methods that copy the data both ways at close to hand-written speed. Rename fields, combine several into one (first_name + last_name -> name) and split them back out, and skip fields with conditions on the source, the destination or the mapping itself. Works with any objects that have getters and setters: Structs, ActiveRecord models, plain Ruby classes.