如何限制MongoDB中的数组子元素？
您可以使用$slice运算符来限制数组。让我们创建包含文档的集合。以下是查询
> db.limitAnArrayDemo.insertOne(
... {
... _id: 101,
... "PlayerName": "Bob",
... "PlayerDetails": {Age:23,isStudent:true},
... "PlayerGameScores": [234,5767,58,67,78,90,1002,576,68,45,23,45,678,89,78 ]
... }
... );
{ "acknowledged" : true, "insertedId" : 101 }以下是在find()方法的帮助下显示集合中所有文档的查询
> db.limitAnArrayDemo.find().pretty();
这将产生以下输出
{
"_id" : 101,
"PlayerName" : "Bob",
"PlayerDetails" : {
"Age" : 23,
"isStudent" : true
},
"PlayerGameScores" : [
234,
5767,
58,
67,
78,
90,
1002,
576,
68,
45,
23,
45,
678,
89,
78
]
}以下是限制MongoDB中的数组子元素的查询。以下查询将仅显示最后10个元素
> db.limitAnArrayDemo.find({}, {PlayerName:1, PlayerDetails: 1, PlayerGameScores:{$slice: -10}}).pretty();以下是显示10个PlayerGameScores的输出
{
"_id" : 101,
"PlayerName" : "Bob",
"PlayerDetails" : {
"Age" : 23,
"isStudent" : true
},
"PlayerGameScores" : [
90,
1002,
576,
68,
45,
23,
45,
678,
89,
78
]
}