MongoDB oplog has entries with dots in key names that cannot be queried, afaict

Considering: Mongo allows you to set nested fields using "dots", for example:

rs0:PRIMARY> db.tmp.update({ a: 1 }, { $set: { 'b.c': 2 } }, { upsert: true })
rs0:PRIMARY> db.tmp.findOne()
{
    "_id" : ObjectId("558251c6a3354af70d70f3cc"),
    "a" : 1,
    "b" : {
        "c" : 2
    }
}

      

        
        
        
      

    

In this example, the entry was created with upsert, which I can check in the oplog:

rs0:PRIMARY> use local
rs0:PRIMARY> db.oplog.rs.find().sort({ts:-1}).limit(1).pretty()
{
    "ts" : Timestamp(1434603974, 2),
    "h" : NumberLong("2071516013149720999"),
    "v" : 2,
    "op" : "i",
    "ns" : "test.tmp",
    "o" : {
        "_id" : ObjectId("558251c6a3354af70d70f3cc"),
        "a" : 1,
        "b" : {
            "c" : 2
        }
    }
}

      

        
        
        
      

    

When I do the same and the entry is just updated rather than created, I seem to get the same behavior:

rs0:PRIMARY> db.tmp.update({ a: 1 }, { $set: { 'b.d': 3 } }, { upsert: true })
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
rs0:PRIMARY> db.tmp.findOne()
{
    "_id" : ObjectId("558251c6a3354af70d70f3cc"),
    "a" : 1,
    "b" : {
        "c" : 2,
        "d" : 3
    }
}

      

        
        
        
      

    

However , this time the oplog entry is structured differently:

rs0:PRIMARY> use local
rs0:PRIMARY> db.oplog.rs.find().sort({ts:-1}).limit(1).pretty()
{
    "ts" : Timestamp(1434604173, 1),
    "h" : NumberLong("-4353495487634403370"),
    "v" : 2,
    "op" : "u",
    "ns" : "test.tmp",
    "o2" : {
        "_id" : ObjectId("558251c6a3354af70d70f3cc")
    },
    "o" : {
        "$set" : {
            "b.d" : 3
        }
    }
}

      

        
        
        
      

    

(Note the key "b.d"

).

This is causing me problems because I am trying to investigate some of the dropped updates by checking the corresponding oplog entries, but AFAICT there is no way to query the oplog entries that set specific nested fields:

rs0:PRIMARY> db.oplog.rs.findOne({ 'o.$set.b.d': { $exists: true } })
null

      

        
        
        
      

    

Is there a way to query the oplog for entries related to updates to a specific subfield (in this case b.d

)?

It seems that I am running into Mongo's inconsistent application of disallowing dots in field names : on the one hand, I cannot create (via the official clients / directly in the Mongo shell) or query for them, but on the other hand it creates them in the oplog, leaving unloadable oplog entries.

Any help would be much appreciated.

For completeness, please note that I can successfully query oplog entries with keys that include the bit $set

:

rs0:PRIMARY> db.tmp.update({ a: 1 }, { $set: { e: 4 } }, { upsert: true })
rs0:PRIMARY> use local
rs0:PRIMARY> db.oplog.rs.findOne({ 'o.$set.e': { $exists: true } })
{
    "ts" : Timestamp(1434604486, 1),
    "h" : NumberLong("1819316318253662899"),
    "v" : 2,
    "op" : "u",
    "ns" : "test.tmp",
    "o2" : {
        "_id" : ObjectId("558251c6a3354af70d70f3cc")
    },
    "o" : {
        "$set" : {
            "e" : 4
        }
    }
}