Mongoose schema reference and undefined type 'ObjectID'
mongoose.Types.ObjectId is the ObjectId constructor function, what you want to use in schema definitions is mongoose.Schema.Types.ObjectId (or mongoose.Schema.ObjectId).
So deviceSchema should look like this instead:
var deviceSchema = schema({ name : String, type : String, room: {type: mongoose.Schema.Types.ObjectId, ref: 'Room'}, users: [{type:mongoose.Schema.Types.ObjectId, ref: 'User'}]});