mongodb TTL not removing documents

Question

I have a simple schema like:

{
    _id: String,      // auto generated
    key: String,      // there is a unique index on this field
    timestamp: Date() // set to current time
}

Then I set the TTL index like so:

db.sess.ensureIndex( { "timestamp": 1 }, { expireAfterSeconds: 3600 } )

I expect the record to removed after 1 hour but it is never removed. I flipped on verbose logging and I see the TTLMonitor running:

Tue Sep 10 10:42:37.081 [TTLMonitor] TTL: { timestamp: 1.0 } { timestamp: { $lt: new Date(1378823557081) } }
Tue Sep 10 10:42:37.081 [TTLMonitor] TTL deleted: 0

When I run that query myself I see all my expired records coming back:

db.sess.find({ timestamp: { $lt: new Date(1378823557081) }})

...

Any ideas? I'm stumped.

EDIT - Example document below

{ "_id" : "3971446b45e640fdb30ebb3d58663807", "key" : "6XTHYKG7XBTQE9MJH8", "timestamp" : ISODate("2013-09-09T18:54:28Z") }

Answer 1

This was my issue: I had the index created wrong like this:

{
    "v" : 1,
    "key" : {
        "columnName" : 1,
        "expireAfterSeconds" : 172800
    },
    "name" : "columnName_1_expireAfterSeconds_172800",
    "ns" : "dbName.collectionName"
}

When it should have been this: (expireAfterSeconds is a top level propery)

{
    "v" : 1,
    "key" : {
        "columnName" : 1
    },
    "expireAfterSeconds" : 172800,
    "name" : "columnName_1_expireAfterSeconds_172800",
    "ns" : "dbName.collectionName"
}

Answer 2

1. Can you show us what the inserted records actually look like?

2. How long is "never"? Because there's a big warning:

   Warning: The TTL index does not guarantee that expired data will be deleted immediately. There may be a delay between the time a document expires and the time that MongoDB removes the document from the database.

3. Does the timestamp field have an index already?