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式是什么?
What is the best way to return all documents in a collection if I want document.a == document.b?
我试过了
db.collection.aggregate([ { $match: { $eq: [ '$a', '$b' ] } }])
但它返回时没有错误或结果,因为我认为它实际上是匹配字符串$a"和$b".是否有不同的方式来指定这些是字段?
But it returns with no errors or results, because I assume it is literally matching strings "$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 } }])
上述方法可行,但需要额外的步骤,即使用它找到的任何文档再次查询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.
有没有更好的方法来实现这个查询?
Is there a better way for achieving this query?
推荐答案
基本上,您正在尝试执行自联接.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
操作符,你猜到了:
Concerning the $eq
operator, as you guessed:
- 根据设计,
$eq
比较查询运算符将字段与值匹配. - 但是
$eq
比较聚合算子 比较两个表达式的值.
- 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.
除了按照您的建议使用额外的 $project
步骤之外,我不知道还有其他方法可以执行您需要的操作.
I don't know any other way to perform what you need than using an extra $project
step as you suggested.
请注意,这并没有显着更昂贵,因为无论如何,您的查询不能使用任何索引,MongoDB 将执行完整扫描.
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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