Model Data for Atomic Operations


Consider the following example that keeps a library book and its checkout information. The example illustrates how embedding fields related to an atomic update within the same document ensures that the fields are in sync.

Consider the following book document that stores the number of available copies for checkout and the current checkout information:

book = {
          _id: 123456789,
          title: "MongoDB: The Definitive Guide",
          author: [ "Kristina Chodorow", "Mike Dirolf" ],
          published_date: ISODate("2010-09-24"),
          pages: 216,
          language: "English",
          publisher_id: "oreilly",
          available: 3,
          checkout: [ { by: "joe", date: ISODate("2012-10-15") } ]

You can use the db.collection.findAndModify() method to atomically determine if a book is available for checkout and update with the new checkout information. Embedding the available field and the checkout field within the same document ensures that the updates to these fields are in sync:

db.books.findAndModify ( {
   query: {
            _id: 123456789,
            available: { $gt: 0 }
   update: {
             $inc: { available: -1 },
             $push: { checkout: { by: "abc", date: new Date() } }
} )