我想得到本地字段特定projectId vise的计数 . 例如我有以下文件:
{
"_id" : ObjectId("5c0a4efa91b5021228681f7a"),
"projectId" : ObjectId("5c0a4083753a321c6c4ee024"),
"hours" : 8,
"__v" : 0
}
{
"_id" : ObjectId("5c0a4f4191b5021228681f7c"),
"projectId" : ObjectId("5c0a2a8897e71a0d28b910ac"),
"hours" : 6,
"__v" : 0
}
{
"_id" : ObjectId("5c0a4f4191b5021228681f7d"),
"projectId" : ObjectId("5c0a4083753a321c6c4ee024"),
"hours" : 2,
"__v" : 0
}
现在,我想让projectId的小时字段数等于 5c0a4083753a321c6c4ee024 ,即10 .
Is it possible with $lookup?
1 回答
$lookup用于连接两个集合 . 在这里,您试图获得特定
projectId
的总计数hours
. 因此,您需要在此处使用$group .