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I have a query built around a composite index, and I've verified from the mongo shell that this query can run as an index only query.

Output:

db.merchants.find({st:{"$regex" : "cr", "$options": "i"}}, {n:1, _id:1})
{ "n" : "Crab shack", "_id" : ObjectId("51c908cc1925f3ca51000001") }
{ "n" : "Tacos R Us", "_id" : ObjectId("51c16f201925f3df4300001b") }
db.merchants.find({st:{"$regex" : "cr", "$options": "i"}}, {n:1, _id:1}).explain()
{
    "cursor" : "BtreeCursor st_1_n_1__id_1 multi",
    "isMultiKey" : false,
    "n" : 2,
    "nscannedObjects" : 2,
    "nscanned" : 2,
    "nscannedObjectsAllPlans" : 2,
    "nscannedAllPlans" : 2,
    "scanAndOrder" : false,
    "indexOnly" : true,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "st" : [
            [
                "",
                {

                }
            ],
            [
                /cr/i,
                /cr/i
            ]
        ],
        "n" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ],
        "_id" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    },
    "server" : "MacBook-Pro.local:27017"
}

But the same query run from the rails console using mongoid gives me:

Merchant.where({:st => { '$regex'=> "cr", '$options' => 'i' } }).only(:_id, :n).explain
=> {
    "cursor"=>"BtreeCursor st_1_n_1__id_1 multi",
    "isMultiKey"=>false, 
    "n"=>2, 
    "nscannedObjects"=>2, 
    "nscanned"=>2, 
    "nscannedObjectsAllPlans"=>2, 
    "nscannedAllPlans"=>2, 
    "scanAndOrder"=>false, 
    "indexOnly"=>false, 
    "nYields"=>0, 
    "nChunkSkips"=>0, 
    "millis"=>0, 
    "indexBounds"=>{
        "st"=>[["", {}], [/cr/i, /cr/i]], 
        "n"=>[[{"$minElement"=>1}, {"$maxElement"=>1}]], "_id"=>[[{"$minElement"=>1}, {"$maxElement"=>1}]]
    }, 
    "allPlans"=>[{"cursor"=>"BtreeCursor st_1_n_1__id_1 multi", "n"=>2, "nscannedObjects"=>2, "nscanned"=>2, "index
Bounds"=>{"st"=>[["", {}], [/cr/i, /cr/i]], "n"=>[[{"$minElement"=>1}, {"$maxElement"=>1}]], "_id"=>[[{"$minElement"=>1}, {"$maxElement"=>1}]]}}], "oldPlan"=>{"cursor"=>"Btr
eeCursor st_1_n_1__id_1 multi", "indexBounds"=>{"st"=>[["", {}], [/cr/i, /cr/i]], "n"=>[[{"$minElement"=>1}, {"$maxElement"=>1}]], "_id"=>[[{"$minElement"=>1}, {"$maxElement
"=>1}]]}}, "server"=>"Arvinds-MacBook-Pro.local:27017"}

I suspect this is because of the only() modifier. Or is this a bug?

EDIT: I'm 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!

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2 回答 2

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我不确定是否仅在将查询发送到 MongoDB 时才实际设置投影。如果您检查日志（在将日志设置得足够高以显示所有查询之后），您应该看到实际发送了哪个查询，包括“投影”。这将向您显示 Mongoid 是否实际发送它，或者只是在应用程序层中解释“仅”。

于 2013-07-12T08:56:18.760 回答
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将日志中发送到 MongoDB 的查询与未调用的相同查询进行比较explain。在 MongoDB 2.2 / Mongoid 3.1.4 上，我看到发送到数据库的正确查询。在轻便摩托车日志中查找fields=指令。

于 2013-09-27T14:41:42.427 回答