> db.activities.find()
{ "_id" : ObjectId("4cd8a3d15c5973397c000027"), "extra" : { "public" : true }, "time" : 1289266129.758516, "actor" : "user", "user" : "4cd714cd5c59731c7a000025", "criteria" : "instance", "message" : [
        {
                "display" : "",
                "id" : "4cd714cd5c59731c7a000025",
                "entity" : "user"
        },
        {
                "display" : "just created a otherstuff called",
                "entity" : "text"
        },
        {
                "display" : "",
                "id" : "4cd8a3d15c5973397c000025",
                "entity" : "otherstuff"
        }
], "type" : "otherstuffs.created" }
{ "_id" : ObjectId("4cd8a3d35c5973397c00002a"), "extra" : { "public" : true }, "time" : 1289266131.998971, "actor" : "user", "user" : "4cd714cd5c59731c7a000025", "criteria" : "instance", "message" : [
        {
                "display" : "",
                "id" : "4cd714cd5c59731c7a000025",
                "entity" : "user"
        },
        {
                "display" : "is a otherstuff creation machine - now we have otherstuff",
                "entity" : "text"
        },
        {
                "display" : "",
                "id" : "4cd8a3d35c5973397c000028",
                "entity" : "otherstuff"
        }
], "type" : "otherstuffs.created" }

> db.activities.getIndexes()
[
        {
                "name" : "_id_",
                "ns" : "realdeal.activities",
                "key" : {
                        "_id" : 1
                }
        },
        {
                "_id" : ObjectId("4cd8a586e2c8302e52577808"),
                "ns" : "realdeal.activities",

                "key" : {
                        "user" : 1,
                        "actor" : 1,
                        "extra.public" : 1,
                        "time" : -1
                },
                "name" : "user_1_actor_1_extra.public_1_time_-1"
        }
]

Here is the query... (which I run via PyMongo)...

{'$or': [{'extra.public': True}, {'extra.otherstuff': {'$in': []}, 'extra.public': False}, {'extra.otherstuff': {'$in': [u'4cd8a3d15c5973397c000025', u'4cd8a3d35c5973397c000028']}, 'extra.public':
False}, {'extra.otherstuff': {'$in': []}, 'extra.public': False}], 'user': {'$in': []}, 'actor': 'user'}

Here is the sort order...

[('time', -1)]

Here is the explain output...

{u'allPlans': [{u'cursor': u'BasicCursor', u'indexBounds': {}}], u'millis': 0,
u'n': 0, u'cursor': u'BasicCursor', u'indexBounds': {}, u'nscannedObjects': 0, u'nscanned': 0}