Question

I’m using MongoDB to hold a collection of documents.

Each document has an _id (version) which is an ObjectId. Each document has a documentId which is shared across the different versions. This too is an OjectId assigned when the first document was created.

What’s the most efficient way of finding the most up-to-date version of a document given the documentId?

I.e. I want to get the record where _id = max(_id) and documentId = x

Do I need to use MapReduce?

Thanks in advance,

Sam

Answer 1

Add index containing both fields (documentId, _id) and don’t use max (what for)? Use query with documentId = x, order DESC by _id and limit(1) results to get the latest. Remember about proper sorting order of index (DESC also)

Something like that

db.collection.find({documentId : "x"}).sort({_id : -1}).limit(1)

Other approach (more denormalized) would be to use other collecion with documents like:

{
    documentId : "x",
    latestVersionId : ...
}

Use of atomic operations would allow to safely update this collection. Adding proper index would make queries fast as lightning.

There is one thing to take into account – i’m not sure whether ObjectID can always be safely used to order by for latest version. Using timestamp may be more certain approach.