问题描述
假设我有一个文档集合,其中文档的ratio属性为浮点数.
Let's assume I have a collection with documents with a ratio attribute that is a floating point number.
{'ratio':1.437}
如何编写查询以查找具有最接近给定整数值的单个文档,而又不使用驱动程序将它们全部加载到内存中并查找具有最小值abs(x-ratio)
的单个文档?
How do I write a query to find the single document with the closest value to a given integer without loading them all into memory using a driver and finding one with the smallest value of abs(x-ratio)
?
推荐答案
有趣的问题.我不知道您是否可以在单个查询中做到这一点,但是可以在两个查询中做到这一点:
Interesting problem. I don't know if you can do it in a single query, but you can do it in two:
var x = 1; // given integer
closestBelow = db.test.find({ratio: {$lte: x}}).sort({ratio: -1}).limit(1);
closestAbove = db.test.find({ratio: {$gt: x}}).sort({ratio: 1}).limit(1);
然后,您只需检查两个文档中哪个文档的ratio
最接近目标整数即可.
Then you just check which of the two docs has the ratio
closest to the target integer.
MongoDB 3.2更新
3.2版本增加了对 $abs
绝对值聚合运算符,该运算符现在允许在单个aggregate
查询中完成此操作:
The 3.2 release adds support for the $abs
absolute value aggregation operator which now allows this to be done in a single aggregate
query:
var x = 1;
db.test.aggregate([
// Project a diff field that's the absolute difference along with the original doc.
{$project: {diff: {$abs: {$subtract: [x, '$ratio']}}, doc: '$$ROOT'}},
// Order the docs by diff
{$sort: {diff: 1}},
// Take the first one
{$limit: 1}
])
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