MongoDB atomic “findOrCreate”: findOne, insert if nonexistent, but do not update

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Beginning with MongoDB 2.4, it’s no longer necessary to rely on a unique index (or any other workaround) for atomic findOrCreate like operations.

This is thanks to the $setOnInsert operator new to 2.4, which allows you to specify updates which should only happen when inserting documents.

This, combined with the upsert option, means you can use findAndModify to achieve an atomic findOrCreate-like operation.

db.collection.findAndModify({
  query: { _id: "some potentially existing id" },
  update: {
    $setOnInsert: { foo: "bar" }
  },
  new: true,   // return new doc if one is upserted
  upsert: true // insert the document if it does not exist
})

As $setOnInsert only affects documents being inserted, if an existing document is found, no modification will occur. If no document exists, it will upsert one with the specified _id, then perform the insert only set. In both cases, the document is returned.

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