node.js - MongoDB atomic "findOrCreate": findOne, insert if nonexistent, but do not update

Question

as the title says, I want to perform a find (one) for a document, by _id, and if doesn't exist, have it created, then whether it was found or was created, have it returned in the callback.

I don't want to update it if it exists, as I've read findAndModify does. I have seen many other questions on Stackoverflow regarding this but again, don't wish to update anything.

I am unsure if by creating (of not existing), THAT is actually the update everyone is talking about, it's all so confuzzling :(

Answer 1

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.