在 Mongodb 中按值乘以字段
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Multiply field by value in Mongodb
提问by Brad
I've been looking for a way to create an update statement that will take an existing numeric field and modify it using an expression. For example, if I have a field called Price, is it possible to do an update that sets Price to 50% off the existing value ?
我一直在寻找一种方法来创建更新语句，该语句将采用现有数字字段并使用表达式对其进行修改。例如，如果我有一个名为 Price 的字段，是否可以进行更新，将 Price 设置为现有值的 50% ？
So, given { Price : 19.99 }
所以，给定 { Price : 19.99 }
I'd like to do db.collection.update({tag : "refurb"}, {$set {Price : Price * 0.50 }}, false, true);
我想做 db.collection.update({tag : "refurb"}, {$set {Price : Price * 0.50 }}, false, true);
Can this be done or do I have to read the value back to the client, modify, then update ? I guess the question then is can expressions be used in update, and can they reference the document being updated.
这可以完成还是我必须将值读回客户端，修改，然后更新？我想问题是可以在更新中使用表达式，并且它们可以引用正在更新的文档。
回答by pingw33n
You can run server-side code with db.eval().
您可以使用db.eval().
db.eval(function() {
db.collection.find({tag : "refurb"}).forEach(function(e) {
e.Price = e.Price * 0.5;
db.collection.save(e);
});
});
Note this will block the DB, so it's better to do find-update operation pair.
注意这会阻塞数据库，所以最好做 find-update 操作对。
See https://docs.mongodb.com/manual/core/server-side-javascript/
请参阅https://docs.mongodb.com/manual/core/server-side-javascript/
回答by Salvador Dali
In the new Mongo 2.6.xthere is a $mul operator. It would multiply the value of the field by the number with the following syntax.
在新的Mongo 2.6.x 中有一个$mul 运算符。它将使用以下语法将字段的值乘以数字。
{
$mul: { field: <number> }
}
So in your case you will need to do the following:
因此，在您的情况下，您需要执行以下操作：
db.collection.update(
{ tag : "refurb"},
{ $mul: { Price : 0.5 } }
);
回答by Xavier Guihot
Starting Mongo 4.2, db.collection.update()can accept an aggregation pipeline, finally allowing the update of a field based on another field:
开始Mongo 4.2，db.collection.update()可以接受一个聚合管道，最后允许基于另一个字段更新一个字段：
// { price: 19.99 }
// { price: 2.04 }
db.collection.update(
{},
[{ $set: { price: { $multiply: [ 0.5, "$price" ] } } }],
{ multi: true }
)
// { price: 9.995 }
// { price: 1.02 }
The first part
{}is the match query, filtering which documents to update (all documents in this case).The second part
[{ $set: { price: ... } }]is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline).$setis a new aggregation operator and an alias of$addFields. Note howpriceis modified directly based on the its own value ($price).Don't forget
{ multi: true }, otherwise only the first matching document will be updated.
第一部分
{}是匹配查询，过滤要更新的文档（在本例中为所有文档）。第二部分
[{ $set: { price: ... } }]是更新聚合管道（注意方括号表示使用聚合管道）。$set是一个新的聚合运算符，是 的别名$addFields。请注意如何price直接根据其自身的值 ($price) 进行修改。不要忘记
{ multi: true }，否则只会更新第一个匹配的文档。
回答by kamaradclimber
Well, this is possible with an atomic operation as $set.
好吧，这可以通过 $set 的原子操作实现。
You have several options :
你有几个选择：
- use the eval() solution proposed by pingw33n
- retrieve the document you want to modify to get the current value and modify it with a set
- if you have a high operation rate, you might want to be sure the focument has not changed during you fetch its value (using the previous solution) so you might want to use a findAndModify (see this pageto get inspired on how to do it) operation.
- 使用 pingw33n 提出的 eval() 解决方案
- 检索要修改的文档以获取当前值并使用集合进行修改
- 如果您的操作率很高，您可能希望确保在获取其值期间焦点没有改变（使用以前的解决方案），因此您可能需要使用 findAndModify（请参阅此页面以获取有关如何操作的灵感） ） 手术。
It really depends on your context : with a very low pressure on the db, I'd go for the solution of pingw33n. With a very high operation rate, I'd use the third solution.
这真的取决于你的上下文：数据库的压力非常低，我会去寻找 pingw33n 的解决方案。由于运行率非常高，我会使用第三种解决方案。
