Разрабатывая ответ Рафаэля Грилли выше,
Правильно:
var HouseSchema = new mongoose.Schema({
date: {type: Date, default:Date.now},
floorplan: String,
name:String,
house_id:String,
addressLine1:String,
addressLine2:String,
city:String,
postCode:String,
_locks:[{type: Schema.Types.ObjectId, ref: 'xxx'}] //ref here refers to the first parameter passed into mongoose.model()
});
var House = mongoose.model('xxx', HouseSchema, 'houseschemas');