CODEWITHSUNDEEP
mongodbfindandmodify
scttnlsn
scttnlsn

Reputation: 3026

MongoDB: Capped collection findAndModify

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

Upvotes: 1

Views: 2036

Answers (1)

Stennie
Stennie

Reputation: 65333

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.

Upvotes: 1

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