Mongo does not have a max() function, how do I work around this?

Question

I have a MongoDB collection and need to find the max() value of a certain field across all docs. This value is the timestamp and I need to find the latest doc by finding th

Answer 1

Try with db.collection.group

For example, with this collection:

> db.foo.find()
{ "_id" : ObjectId("..."), "a" : 1 }
{ "_id" : ObjectId("..."), "a" : 200 }
{ "_id" : ObjectId("..."), "a" : 230 }
{ "_id" : ObjectId("..."), "a" : -2230 }
{ "_id" : ObjectId("..."), "a" : 5230 }
{ "_id" : ObjectId("..."), "a" : 530 }
{ "_id" : ObjectId("..."), "a" : 1530 }

You can use group using

> db.foo.group({
    initial: { },
    reduce: function(doc, acc) {
        if(acc.hasOwnProperty('max')) {
            if(acc.max < doc.a)
                acc.max = doc.a;
        } else {
            acc.max = doc.a
        }
      }
    })
[ { "max" : 5230 } ]

Since there is no key value in group all the objects are grouped in a single result