MongoDB区分未定义和null [英] MongoDB Differentiates Between undefined vs. null

问题描述

我正在检查用于查询非值的逻辑，并注意到使用mongo shell时，它会区分undefined和null值.

I was checking logic for querying on non-values and noticed when using the mongo shell, it differentiates between undefined and null values.

> use test
> db.test.insert({ a : 1,          b : null,       c : undefined })
> db.test.insert({ a : null,       b : undefined,  c : 1 })
> db.test.insert({ a : undefined,  b : 1,          c : null })

查询集合时，会得到以下信息:

When you query on the collection, you get this:

> db.test.find()
{ "_id" : ObjectId("52d95575c9333565e80ccb22"), "a" : 1, "b" : null, "c" : null }
{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }
{ "_id" : ObjectId("52d95586c9333565e80ccb24"), "a" : null, "b" : 1, "c" : null }

但是，当您在null上进行查询时，它仅检索显式设置为null的记录.

When you query on null however, it only retrieves records who was explicitly set to null.

> db.test.find({ a : null })
{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }

这是MongoDB中的错误吗? 如何正确查询null/undefined/non-set字段?

Is this a bug in MongoDB? How can I properly query for null/ undefined/ non-set fields?

因此，我可以使用以下方法查询未设置的值:

So I can query for not-set values with this:

db.test.find({ $or : [ { a : null }, { a : { $exists : false } } ] })

但是在此示例中，它仍然仅返回单个记录:

But in this example though, it still only returns the single record:

{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }

任何人都知道MongoDB为什么/如何区分undefined和null?这与在MongoDB中输入数据的方式有关吗?

Anyone know why/ how MongoDB differentiates between undefined and null? Is this an issue with how the data was entered in MongoDB?

推荐答案

如果要返回存在字段且不为null的文档，请使用{ a : {$ne : null}}

If you want to return a document where a field exists AND is not null, use { a : {$ne : null}}

未定义和null值不同，但是外壳程序将它们都显示为null- https://jira.mongodb.org/browse/SERVER-6102

Undefined and null values are different, but the shell shows them both as null - https://jira.mongodb.org/browse/SERVER-6102

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