MongoDB聚合$ slice以获取数组的长度
为此，请使用$project，然后使用$size来获取长度。首先让我们创建一个包含文档的集合-
> db.demo382.insertOne(
... {
...
... "Name" : "David",
... "details" : [
... {
... "SubjectName":"MySQL"
... },
... {
... "SubjectName":"MongoDB"
... },
... {
... "SubjectName":"Java"
... }
... ]
...
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5b5e1c22064be7ab44e7f0")
}在find()方法＆Minus;的帮助下显示集合中的所有文档。
> db.demo382.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5e5b5e1c22064be7ab44e7f0"),
"Name" : "David",
"details" : [
{
"SubjectName" : "MySQL"
},
{
"SubjectName" : "MongoDB"
},
{
"SubjectName" : "Java"
}
]
}以下是汇总$slice并获取长度的查询-
> db.demo382.aggregate([
... { "$match": { "Name": "David" } },
... { "$project": { "count": { "$size": "$details" }}}
... ])这将产生以下输出-
{ "_id" : ObjectId("5e5b5e1c22064be7ab44e7f0"), "count" : 3 }