Unable to get a covered query to use index only in mongodb - mongodb

Question

I am trying to use a covering index to implement stemming text search on my app which uses mongodb.
I've got the following index set:
ensureIndex({st: 1, n: 1, _id: 1});
But when I run explain() on my query, I can never get the indexOnly to read true, no matter what I do.
db.merchants.find({st: "Blue"}, {n:1,_id:1}).explain()
{
"cursor" : "BtreeCursor st_1_n_1__id_1",
"nscanned" : 8,
"nscannedObjects" : 8,
"n" : 8,
"millis" : 0,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : true,
"indexOnly" : false,
"indexBounds" : {
"st" : [
[
"Blue",
"Blue"
]
],
"n" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"_id" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
}
I've already figured out that the ordering of the keys in the index matter somehow. For instance if I used {_id, n:1, st:1} it wasn't using this index at all to perform the query. I also read somewhere that too few documents could trigger unpredictable behaviour with explain() since multiple strategies are equally fast. But in this case, I see that its using the right index, but its not using just the index. What is this happening?
I am using mongoid, and mongo 2.0.8 I believe.
UPDATE:
Switched over to using Mongoid v3.1.4 and mongod v2.2
Here is the query that mongod is seeing from mongoid: Mon Jul 15 10:47:26 [conn14] runQuery called spl_development.merchants { $query: { st: { $regex: "cr", $options: "i" } }, $explain: true } Mon Jul 15 10:47:26 [conn14] query spl_development.merchants query: { $query: { st: { $regex: "cr", $options: "i" } }, $explain: true } ntoreturn:0 keyUpdates:0 locks(micros) r:212 nreturned:1 reslen:393 0ms
So the projection isn't being sent to the mongod layer and only just handles it in the application layer. Not ideal!
This has been recognized as a bug in mongoid and can be tracked here:
https://github.com/mongoid/mongoid/issues/3142

Answer 1

I expect your query cannot use a covered index because you have a field with an array included in the index. This is suggested in the explain with "isMultiKey" : true.
As noted in the documentation (Create Indexes that Support Covered Queries):
MongoDB cannot use a covered query if any of the indexed fields in any of the documents in the collection includes an array. If an indexed field is an array, the index becomes a multi-key index and cannot support a covered query.

Answer 2

I wasn't able to reproduce the problem in 2.2.2, but add .sort({n: 1, _id: 1}) into the chain. Because you're not sorting, you're asking for the docs in whatever find order mongo wishes to use, and if that doesn't match the order in the index ($natural order, for instance) it still has to read the docs.
db.merchants.find({st: "Blue"}, {n:1,_id:1}).sort({n: 1, _id: 1}).explain()