Residencia eneste ejemplo (que funciona):

var Comment = new Schema();

Comment.add({
    title : { type: String, index: true }
  , date : Date
  , body : String
  , comments : [Comment]
});

Quería crear una versión de CoffeeScript

mongoose = require 'mongoose'
Schema = mongoose.Schema

Person = new Schema
Person.add
  mother: Person
  father: Person

Sin embargo, devuelve un error y no entiendo por qué

TypeError: undefined is not a function
    at CALL_NON_FUNCTION_AS_CONSTRUCTOR (native)
    at Function.interpretAsType (/path/node_modules/mongoose/lib/schema.js:202:10)
    at Schema.path (/path/node_modules/mongoose/lib/schema.js:162:29)
    at Schema.add (/path/node_modules/mongoose/lib/schema.js:110:12)
    at Object.<anonymous> (/path/Models/test.coffee:6:10)
    at Object.<anonymous> (/path/Models/test.coffee:10:4)
    at Module._compile (module.js:411:26)
    at Object.run (/usr/local/lib/node_modules/coffee-script/lib/coffee-script.js:57:25)
    at /usr/local/lib/node_modules/coffee-script/lib/command.js:147:29
    at /usr/local/lib/node_modules/coffee-script/lib/command.js:115:26

EDIT: Bien, descubrí que no funciona cuando Person no es una matriz (entre paréntesis), pero ¿por qué?