thiny.js

// a library
{
  "books": [
    ObjectId(507c7f79bcf86cd7994f6cAB),
    ObjectId(507c7f79bcf86cd7994s320e),
    ObjectId(507c7f79bcf86cd7994s221e)
  ],
  "name": "New York Public Library",
  "_id": ObjectId(507c7f7921sa6cd7994s22LB)
}

// a book
{
  "title": "Moby Dick",
  "author": "Herman Melville",
  "library": ObjectId(507c7f7921sa6cd7994s22LB),
  "_id": ObjectId(507c7f79bcf86cd7994f6cAB)
}