[英]Update to average value in Mongo document with pymongo
I want to update field in exists mongodb document. 我想更新存在的mongodb文档中的字段。 But set value should be average value of old value and new value.
但设定值应为旧值和新值的平均值。
I can get old value from mongodb document and calculate average value, and set into field, but it's not thread safety and old value in mongodb document might change while I calculate average. 我可以从mongodb文档中获取旧值并计算平均值,然后设置为字段,但它不是线程安全性,因此在计算平均值时,mongodb文档中的旧值可能会更改。
Example, document: 示例文件:
{ '_id': ObjectId("55d49338b9796c337c894df3"), value: 10 }
{'_id':ObjectId(“ 55d49338b9796c337c894df3”),值:10}
Python code: Python代码:
# new_value = 15, therefore average_value = 12.5
db.mycollection.update_one(
{'_id': '55d49338b9796c337c894df3'},
{...} <- What there?
)
Before: 之前:
{ '_id': ObjectId("55d49338b9796c337c894df3"), value: 12.5 }
{'_id':ObjectId(“ 55d49338b9796c337c894df3”),值:12.5}
You can use the aggregation framework to do update. 您可以使用聚合框架进行更新。 The pipeline steps you need for this are
$addFields
and $out
. 为此所需的管道步骤是
$addFields
和$out
。 The $addFields
operator allows you to replace an existing fields within the collection with results from an expression, which would involve the arithmetic operator $avg
to compute the average. $addFields
运算符使您可以用表达式的结果替换集合中的现有字段,该表达式将涉及算术运算符$avg
以计算平均值。
The $avg
operator, if used in the $project
(or $addFields
) stage, can accept a list of expressions and with that list you can push values from the existing field and the new value for computing the average of these two. $avg
运算符(如果在$project
(或$addFields
)阶段中使用)可以接受表达式列表,使用该列表,您可以从现有字段中推入值,并推入新值以计算这两个值的平均值。
The $out
operator as the final stage will update the existing collection as it writes the resulting documents of the aggregation pipeline to a collection. 作为最后阶段的
$out
运算符将更新现有集合,因为它将聚合管道的结果文档写入到集合中。
The following example describes the above update operation disguised as an aggregate operation: 以下示例描述了伪装成聚合操作的上述更新操作:
new_value = 15
db.mycollection.aggregate([
{ "$addFields": {
"value": { "$avg": ["$value", new_value] }
} },
{ "$out": "mycollection" }
])
or with MongoDB 3.2 using $project
as 或在MongoDB 3.2中使用
$project
作为
new_value = 15
db.mycollection.aggregate([
{ "$project": {
"value": { "$avg": ["$value", new_value] }
} },
{ "$out": "mycollection" }
])
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