Compound index by array in mongo - mongodb

Question

I have a mongodb collection where documents contain the following fields (documents have a lot of fields but I removed them for understanding):
{
"_id": ObjectId("53aad11444d0e2fd648b4567"),
"id": NumberLong(238790),
"rid": NumberLong(12),
"parent_id": {
"0": NumberLong(12),
"1": NumberLong(2)
},
"coid": NumberLong(3159),
"reid": NumberLong(4312),
"cid": NumberLong(4400)
}
When I run a query I get
> db.ads2.find({coid:3159, parent_id : 2}).sort({inserdate:1}).explain()
{
"cursor" : "BtreeCursor coid_1_parent_id_1_insertdate_-1",
"isMultiKey" : true,
"n" : 20444,
"nscannedObjects" : 20444,
"nscanned" : 20444,
"nscannedObjectsAllPlans" : 20444,
"nscannedAllPlans" : 20444,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 319,
"nChunkSkips" : 0,
"millis" : 274,
"indexBounds" : {
"coid" : [
[
3159,
3159
]
],
"parent_id" : [
[
2,
2
]
],
"insertdate" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
},
"server" : "myserver.com:27017",
"filterSet" : false
}
The question is how should I change the index to make mongo to use it properly?

Answer 1

First of all, your query and the document structure you posted do not match. If you run this query on the collection that contains documents with that kind of structure you will get no results.
But since your query yielded 20444 results I guess that your structure actually looks like this:
{
...
"parent_id": [
"0": NumberLong(12),
"1": NumberLong(2)
],
...
}
This is the index you created:
db.x.ensureIndex({coid : 1, parent_id : 1, insertdate : -1});
From your explain() output you can see that MongoDB is using index to find the documents (because number of scanned documents is equal to the number of returned documents n == nscanned). But, "scanAndOrder" : true means that MongoDB is not using the index to sort your documents.
Your index should be used for matching and sorting if the field you're using to sort exist in the documents.
But the problem is that I can't see insertdate field nowhere in your structure. So if you're sorting by a field that doesn't exist in your document, normally, MongoDB can't use the index for sorting.
Edit
After your comment and examination of your original question I probably found what's causing your problem. You have a typo in the query you're executing. You're specifying the sort parameter as inserdate, while the name of the field that was indexes is insertdate.