如何通过 MongoDB 聚合框架中的“未定义”值进行匹配？
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How to match by 'undefined' value in MongoDB Aggregation Framework?
提问by BreakPhreak
How can I search for the records filtering ina field that has an undefinedvalue:
如何在具有undefined值的字段中搜索记录过滤：
db.records.aggregate({
$match: {
myField: "undefined",
}
})
回答by user854301
Filter it by $type:6, (mongodb referece, note that this type marked as 'deprecated'):
过滤它$type:6，（mongodb 引用，注意这个类型被标记为 'deprecated'）：
db.records.aggregate({
$match: {
myField: {'$type':6},
}
})
回答by ixe013
If you want to filter out documents that have some fields missing, use the $existsoperator.
如果要过滤掉缺少某些字段的文档，请使用$exists运算符。
This works on my machine :
这适用于我的机器：
> db.test.drop()
true
> db.test.insert( {'Hello':'World!', 'myField':42})
> db.test.insert( {'Hello again':'World!'})
> db.test.aggregate({'$match':{ 'myField':{'$exists':false} }})
{
"result" : [
{
"_id" : ObjectId("51b9cd2a6c6a334430ec0c98"),
"Hello again" : "World!"
}
],
"ok" : 1
}
The document that has myField present does not show in the results.
存在 myField 的文档不会显示在结果中。
