Mongoose Models in Express App (not found by require)

I am trying to define a simple Mongoose model for a learning ExpressJS app.

# Todo.coffee

mongoose = require "mongoose"
Schema = mongoose.Schema

todoSchema = new Schema 
    title: String
    desc: String
    dueOn: Date
    completedOn: Date

Todo = new mongoose.model("Todo", todoSchema)

I have a Todos class that is meant for other classes to use. (Not sure if its better to just use the Todo model tho?)

# Todos.coffee

Todo = require "Todo"

class Todos 

    byId: (id, callback) ->
        Todo.findOne { _id: id }, callback

Then in my Mocha/should unit test ...

require "should"
Todos = require "../../../app/todos/Todos.coffee"
Todo = require "../../../app/todos/Todo.coffee"

describe "Todos", ->
    describe "byId", ->
        it "should return Todo with given ID", (done) ->
            Todos.byId 1, (err, todo) ->
                todo.should.be.an.instanceOf(Todo)
                done()

But I am getting ...

$ mocha --compilers coffee:coffee-script tests/backend/todos/TodosTests.coffee 

module.js:340
    throw err;
          ^
Error: Cannot find module 'Todo'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:362:17)
    at require (module.js:378:17)
    at Object.<anonymous> (/labs/Projects/Tickle/app/todos/Todos.coffee:4:10)
    at Object.<anonymous> (/labs/Projects/Tickle/app/todos/Todos.coffee:20:4)
    at Module._compile (module.js:449:26)
    at Object.require.extensions..coffee (/usr/lib/node_modules/coffee-script/lib/coffee-script/coffee-script.js:22:21)

What did I do wrong?