MongoDB:查询具有两个相等字段($ match和$ eq)的文档 [英] MongoDB : querying documents with two equal fields, $match and $eq

问题描述

如果我想要document.a == document.b，返回集合中所有文档的最佳方法是什么?

What is the best way to return all documents in a collection if I want document.a == document.b?

我尝试过

db.collection.aggregate([ { $match: { $eq: [ '$a', '$b' ] } }])

但是它返回没有错误或结果，因为我假设它实际上是匹配字符串"$ a"和"$ b"的字符串.是否有其他方法可以指定这些字段?

But it returns with no errors or results, because I assume it is literally matching strings "$a" and "$b". Is there a different way to specify that these are fields?

db.collection.aggregate([    { $project: { 
    eq: { $cond: [ { $eq: [ '$a', '$b' ] }, 1, 0 ] } 
} },
{ $match: { eq: 1 } }])

上面的方法有效，但是需要额外的步骤，即再次查询找到的任何文档或投影所有可能的字段.

The above works, but requires the additional step of querying again with whatever documents it found or projecting all possible fields.

是否有更好的方法来实现此查询?

Is there a better way for achieving this query?

推荐答案

基本上，您正在尝试执行自我连接. MongoDB不支持的操作.

Basically, you are trying to perform a self join. An operation not supported by MongoDB.

关于$eq运算符，正如您所猜测的那样:

Concerning the $eq operator, as you guessed:

• 根据设计， $eq比较查询运算符匹配针对 value 的字段.
• 但是 $eq比较聚合运算符比较该值两个表达式中的一个.

• By design, the $eq comparison query operator match a field against a value.
• But the $eq comparison aggregation operator compare the value of two expressions.

除了您建议使用额外的$project步骤之外，我不知道其他任何方法来执行所需的操作.

I don't know any other way to perform what you need than using an extra $project step as you suggested.

请注意，这并不昂贵，因为无论如何，您的查询不能使用任何索引，MongoDB会进行全面扫描.

Please note this is not significantly more expensive as, anyway, your query cannot use any index and MongoDB will do a full scan.

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