mongodb 字段“$_id”必须是累加器对象
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/44042329/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
The Field "$_id" must be an accumulator object
提问by ffritz
I am trying to groupsome documents within mongoDB, right after a $matchstage like this:
我正在groupmongoDB 中尝试一些文档，就在这样的$match阶段之后：
db.trips.aggregate([
{ "$match": {
"Stop Time": {
"$lt": "31.07.2013 23:59"
}
}
},
{ "$group": {
"$_id": { "Start": "$Start", "End": "$End" },
}
}]
However I am getting the following error:
但是我收到以下错误：
"The field '$_id' must be an accumulator object"
“字段‘$_id’必须是累加器对象”
Why does this error show up, and why does it only show up when I include the match stage? Without the match stage, it works just fine.
为什么会出现此错误，为什么仅在我包含比赛阶段时才会出现？没有比赛阶段，它工作得很好。
回答by user3317755
$_idis not an accumulator function it should be _id
$_id不是累加器函数，它应该是_id
