MongoDb TTL on nested document is possible?

问题:

I want to know if it's possible to use TTL on nested documents.

Scenario

I have Account and inside I have Sessions. Sessions need to expire in 30 minutes. I've set everything up but obviously when I set TTL index on Account.Sessions.EndDateTime it removes the whole Account. Can I make sure it removes only Session?

This is what it looks like in database. Notice how it will delete whole Account and not only Session when EndDateTime will come.

{
    "_id" : ObjectId("53af273888dba003f429540b"),
    "Email" : "[e-mail]",
    "PasswordHash" : "CZaBEQRbwWNgJBjyhks7gH0Z3v5ZvDkW29pryF0DEXyE8rIw0NA4x39+uQneArKaUv97sP1e+e22YT1glbqQsw==",
    "PasswordSalt" : "100000.Qx4D8uj7oDcWHRTLGRRTDwVkw2UcaM52XkDR9k2ga073Ow==",
    "Sessions" : [ 
        {
            "Token" : "da55cf0783c4249b26283948fcae6caa15df320ca456203045aea81cad691df8",
            "IpAddress" : "::1",
            "StartDateTime" : ISODate("2014-06-28T20:36:27.000Z"),
            "EndDateTime" : ISODate("2014-06-28T21:06:27.000Z")
        }
    ]
}

This is where I create said index.

if (!_db.Accounts.IndexExists("Sessions.EndDateTime"))
{
    _db.Accounts.CreateIndex(IndexKeys.Ascending("Sessions.EndDateTime"),
        IndexOptions.SetTimeToLive(new TimeSpan(0)));
}

回答1:

That is currently not possible with TTL index. Mongod will remove the whole document after a specified number of seconds or at a specific clock time.

TTL relies on a background thread in mongod that reads the date-typed values in the index and removes expired documents from the collection.

I would recommend that you store the session sub-document in a separate collection and add a TTL index on that collection.

If you can't change your schema, the alternative is to create a background job that will delete nested documents from your collection every 60 seconds.