MongoDB TTL sparse index?

Question

Can you TTL a sparse field? If so, should you declare the TTL index sparse? Like so?

db.eventlog.createIndex( { "lastModifiedDate": 1 }, { expireAfterSeconds: 3600 , sparse:"true"} )

Answer 1

> use foo
switched to db foo
> db.foo.createIndex({date: 1}, {expireAfterSeconds: 5, sparse: true})
{
    "createdCollectionAutomatically" : false,
    "numIndexesBefore" : 1,
    "numIndexesAfter" : 2,
    "ok" : 1
}

> db.foo.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "foo.foo"
    },
    {
        "v" : 1,
        "key" : {
            "date" : 1
        },
        "name" : "date_1",
        "ns" : "foo.foo",
        "expireAfterSeconds" : 5,
        "sparse" : true
    }
]

> db.foo.insert({date: new Date()})

> db.foo.find()
{ "_id" : ObjectId("5841aeb650b5412e92ebbb9b"), "date" : ISODate("2016-12-02T17:26:14.617Z") }

> db.foo.find()
>

It appears that this worked fine. Note that according to the documentation , the TTL operation fires every 60 seconds or so, so the expireAfterSeconds: 5 may take longer.