【发布时间】：2016-03-05 07:07:14
【问题描述】：
我正在尝试找到一种有效的方法来搜索特定字段为 null 的项目。
在 MongoDB 中，我有 folder 架构，它的父文件夹的 ObjectId 和 parent 的 parent 字段已被索引。对于根文件夹，父字段为null。
当我尝试使用parent:null 查找所有根文件夹时，explain 显示indexOnly: false
db.folders.find({parent: null}, {parent: 1, _id: 0}).explain()
{
"cursor" : "BtreeCursor parent_1",
"isMultiKey" : false,
"n" : 126,
"nscannedObjects" : 126,
"nscanned" : 126,
"nscannedObjectsAllPlans" : 126,
"nscannedAllPlans" : 126,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 1,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"parent" : [
[
null,
null
]
]
},
"server" : "c268.candidate.36:10268",
"filterSet" : false,
"stats" : {
"type" : "PROJECTION",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "KEEP_MUTATIONS",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "FETCH",
"works" : 128,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"alreadyHasObj" : 0,
"forcedFetches" : 0,
"matchTested" : 126,
"children" : [
{
"type" : "IXSCAN",
"works" : 127,
"yields" : 1,
"unyields" : 1,
"invalidates" : 0,
"advanced" : 126,
"needTime" : 1,
"needFetch" : 0,
"isEOF" : 1,
"keyPattern" : "{ parent: 1.0 }",
"isMultiKey" : 0,
"boundsVerbose" : "field #0['parent']: [null, null]",
"yieldMovedCursor" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0,
"keysExamined" : 126,
"children" : []
}
]
}
]
}
]
}
}
我尝试使用 $type 来查找父文件夹不是 ObjectId 的任何文件夹。但还是indexOnly:false。
db.folders.find({parent: {$ne: {$type: 7}}}, {parent: 1, _id: 0}).explain()
"indexOnly": false
有没有办法只使用索引来搜索空值？如果没有，是否有更好的值来存储而不是 null 以便能够使用索引进行搜索？
附加
根目录示例
{
"_id" : ObjectId("55a04a2d754971030059b7ad"),
"active" : true,
"modified" : ISODate("2016-02-05T22:30:08.053Z"),
"created" : ISODate("2015-07-10T22:41:49.009Z"),
"user" : ObjectId("54d3ae187a738c0300f59e61"),
"name" : "2nd Grade",
"parent" : null,
"clientModified" : ISODate("2016-02-05T22:30:07.872Z"),
"userCreated" : ISODate("2015-07-10T22:41:48.328Z"),
"ancestors" : [],
"__v" : 2
}
子文件夹示例
{
"_id" : ObjectId("56d0b4edb6f05e03009bcabc"),
"active" : true,
"modified" : ISODate("2016-02-26T20:26:21.328Z"),
"created" : ISODate("2016-02-26T20:26:21.328Z"),
"user" : ObjectId("54d3ae187a738c0300f59e61"),
"name" : "music",
"parent" : ObjectId("55a04a2d754971030059b7ad"),
"clientModified" : ISODate("2016-02-26T20:26:20.398Z"),
"userCreated" : ISODate("2016-02-26T20:26:20.398Z"),
"ancestors" : [
ObjectId("55a04a2d754971030059b7ad")
],
"__v" : 0
}
db.folders.getIndexes()
{
"0" : {
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "app29099188.folders"
},
"1" : {
"v" : 1,
"key" : {
"user" : 1,
"_fts" : "text",
"_ftsx" : 1
},
"name" : "user_1_name_text",
"ns" : "app29099188.folders",
"background" : true,
"safe" : null,
"weights" : {
"name" : 1
},
"default_language" : "english",
"language_override" : "language",
"textIndexVersion" : 2
},
"2" : {
"v" : 1,
"key" : {
"user" : 1,
"parent" : 1
},
"name" : "user_1_parent_1",
"ns" : "app29099188.folders",
"background" : true,
"safe" : null
},
"3" : {
"v" : 1,
"key" : {
"parent" : 1.0000000000000000
},
"name" : "parent_1",
"ns" : "app29099188.folders"
}
}
【问题讨论】：
-
能发一下父子文档的例子吗？
-
你也可以发布 db.folders.getIndexes() 吗？
-
顺便说一句 - 你可以使用归零的 ID 代替空条目吗？
-
我编辑的答案有一个父字段不为空的示例
标签： mongodb mongodb-query objectid