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Model Data for Atomic Operations

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In MongoDB, write operations, e.g. db.collection.update(), db.collection.findAndModify(), db.collection.remove(), are atomic on the level of a single document. For fields that must be updated together, embedding the fields within the same document ensures that the fields can be updated atomically.

For example, consider a situation where you need to maintain information on books, including the number of copies available for checkout as well as the current checkout information.

The available copies of the book and the checkout information should be in sync. As such, embedding the available field and the checkout field within the same document ensures that you can update the two fields atomically.

    _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") } ]

Then to update with new checkout information, you can use the db.collection.update() method to atomically update both the available field and the checkout field:

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

The operation returns a WriteResult() object that contains information on the status of the operation:

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

The nMatched field shows that 1 document matched the update condition, and nModified shows that the operation updated 1 document.

If no document matched the update condition, then nMatched and nModified would be 0 and would indicate that you could not check out the book.