Mongo efficient querying log collection by level range - mongodb

Question

I have a capped collection for storing server logs:
var schema = new mongoose.Schema({
level: { type: Number, required: true },
...
}, { capped: 64 * 1024 * 1024, versionKey: false });
I'm having trouble figuring out how to query logs by level range efficiently. Here's a sample query I want to run:
db.getCollection('logs').find({
level: { $gte: 2, $lte: 6 }
}).sort({ _id: -1 }).limit(500)
Indexing on { _id: 1, level: 1 } doesn't make any sense, as _id is unique and there will be only a single level for each of them, so in worst case whole collection will be checked.
If I index on { level: 1, _id: -1 }, in worst case Mongo pulls all logs for levels 2, 3, 4, 5, 6 joins them and sorts them manually, so performance is horrible. Sometimes it also decides to use { _id: 1 } index, which is terrible too.
It could just walk through these 6 indexes at once and get the result while checking at most 504 documents. Or it could pull only first 500 results from each level, so it would sort at most 2500 documents. But it won't, Mongo is just plain stupid when it comes to range queries.
The fastest solution I can think of is implementing the last mentioned method on the client, so running 5 queries and then merging them manually:
db.getCollection('logs').find({ level: 2 }).sort({ _id: -1 }).limit(500)
db.getCollection('logs').find({ level: 2 }).sort({ _id: -1 }).limit(500)
db.getCollection('logs').find({ level: 3 }).sort({ _id: -1 }).limit(500)
...
Merging can be done in O(n) on the client, there are only 7 log levels so at most 7 queries will be executed and 3500 documents pulled from the database.
Is there a better way?

Answer 1

Since you have only 7 levels, it may worth to consider { level: 1, _id: -1 } index with $or query:
db.logs.find({$or:[
{level: 2},
{level: 3},
{level: 4},
{level: 5},
{level: 6}
]}).sort({_id:-1}).limit(500)
Since it is equality condition, it should make use of the index, but I never tried it on capped collections.
I would give it a try and run explain() to confirm it works, then probably enabled profiler and run few other queries.