MongoDB: Capped collection findAndModify

Question

I'm trying to atomically insert an empty document if the capped collection is empty or return the last naturally sorted document if not empty. Can I do this with findAndModify?

db.collection.findAndModify({
    query: { _id: { $exists: true }},
    sort: { $natural: -1 },
    update: {},
    upsert: true,
    new: true
});

I would have expected this to either return the latest document (if the collection is non empty) or insert a new document if none exist, however, it inserts a blank document (without an _id) every single time it's called. Does findAndModify work with capped collections? I need the added document to have an _id.

Thanks.

-Scott

Answer 1

I'm trying to atomically insert an empty document if the capped collection is empty or return the last naturally sorted document if not empty. Can I do this with findAndModify?

There is a flaw in your query logic. A findAndModify() with:

query: { _id: { $exists: true }},
sort: { $natural: -1 },
update: {},
upsert: true,
new: true

... will:

• do an update on the last inserted record with an _id set

  OR

• insert a new (empty) document if no existing document with an _id is found.

The update is going to replace your last inserted record with an empty one .. which presumably is not the intended outcome :).

You are seeing a completely empty document (no _id field) because capped collections have some exceptions to the behaviour for standard collections.

In particular:

• there is no requirement for an _id field by default; you can have one generated on the server by including the autoIndexId:true option to createCollection()

• there is no index on the _id field (note: you will want a unique index if using replication with a capped collection)

Also note that documents in a capped collection must not grow in size or the update will fail

Refer to the Capped Collection Usage & Restrictions on the wiki for more info.