How to query only documents with the latest timestamp from a group?

Question

In the MongoDB collection I\'m querying, each document represents an item at a specific time. When updating a document, a new document is created with the same item id and a

Answer 1

One can consider changing the document's schema to better suit your queries, and reduce the overhead of aggregation. Instead of creating a new document for each revision, you could push revision sub-documents onto an array and maintain the latest revision at the parent document; for example:

{
    _id: x,
    itemId: 123,
    createdOn: ISODate("2014-02-09T14:26:20.102Z"),
    field1: "baz",
    field2: "fiz,
    revisions: [
        {createdOn: ISODate("2013-01-30T11:16:20.102Z"), field1: "foo", field2: "bar"},
        {createdOn: ISODate("2014-02-09T14:26:20.102Z"), field1: "baz", field2: "fiz"}
    ]
}

Keep in mind that MongoDB enforces a document-size limit of 16MB; this should suffice for most use cases. This would make your queries very simple: db.collection.find({field1: "foo"})

Just another approach...

Answer 2

You are mixing a few methods here when you can be doing everything in the aggregation pipeline. Otherwise it's just a matter of getting your steps in the right order:

db.collection.aggregate([
    {$sort: { createdOn: -1 }},
    {$group: { _id: "$itemId", 
        createdOn: {$first: "$createdOn"},
        field1: {$first: "$field1" },
        field2: {$first: "$field2" }
    }},
    {$match: { field1: "foo" }}
])

So sort first for newest documents. Group on the itemId ( order will be maintained for $first ), and then filter with $match if you must. But your grouped documents will be latest ones.