具有空值的 MongoDB 查询
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MongoDB queries with null value
提问by Dewfy
My collection (MongoDB v 2.0.2) has following records:
我的收藏（MongoDB v 2.0.2）有以下记录：
db.organization.find({})
{ "_id" : 1001, "path" : [ ], "parent" : null }
{ "_id" : 1002, "path" : [ 1001 ], "parent" : NumberLong(1001) }
organizationhas indexes:
organization有索引：
db.organization.ensureIndex({"path":1});
db.organization.ensureIndex({"parent":1},{sparse:false});
(note I put awarnes sparse : false- to grant that null is indexed)
But, executing:
（注意我把 awarnes sparse : false- 授予 null 被索引）但是，执行：
db.organization.find({"parent":null})
Returns empty set. What is wrong? Thank you in advance
返回空集。怎么了？先感谢您
回答by Daniel K.
I had the same issue. After reading the following documents
我遇到过同样的问题。阅读以下文件后
I tried to query for the different BSON element types and found that my null was represented as a BSON element type 6 (undefined, deprecated) instead of the expected BSON element type 10 (null).
我尝试查询不同的 BSON 元素类型，发现我的 null 表示为 BSON 元素类型 6（未定义，已弃用），而不是预期的 BSON 元素类型 10（空值）。
db.collection.find({ field: { "$type" : 6} };
回答by Andrew Orsich
Just checked following script at 2.0 and 2.0.2:
刚刚在 2.0 和 2.0.2 中检查了以下脚本：
db.items.insert({ "_id" : 1001, "path" : [ ], "parent" : null })
db.items.insert({ "_id" : 1002, "path" : [ 1001 ], "parent" : NumberLong(1001) })
db.items.ensureIndex({"path":1});
db.items.ensureIndex({"parent":1},{sparse:false});
db.items.find({"parent":null})
actually returns one document that you expect:
实际上返回您期望的一个文档：
{ "_id" : 1001,
"path" : [],
"parent" : null }
Also you can look into this doc about querying and nulls, probably should help you avoid possible future mistakes.
您也可以查看有关querying 和 nulls 的文档，可能会帮助您避免将来可能出现的错误。
