仅在MongoDB中获得数组属性的第一项？
为此使用$slice运算符。首先让我们创建一个包含文档的集合-
> db.gettingFirstItemInArrayDemo.insertOne(
{
"UserId": 101,
"UserName":"Carol",
"UserOtherDetails": [
{"UserFriendName":"Sam"},
{"UserFriendName":"Mike"},
{"UserFriendName":"David"},
{"UserFriendName":"Bob"}
]
}
);
{
"acknowledged" : true,
"insertedId" : ObjectId("5cdfca52bf3115999ed51205")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.gettingFirstItemInArrayDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cdfca52bf3115999ed51205"),
"UserId" : 101,
"UserName" : "Carol",
"UserOtherDetails" : [
{
"UserFriendName" : "Sam"
},
{
"UserFriendName" : "Mike"
},
{
"UserFriendName" : "David"
},
{
"UserFriendName" : "Bob"
}
]
}以下是仅在MongoDB中获取数组属性的第一项的查询-
> db.gettingFirstItemInArrayDemo.find({"UserId":101}, {UserOtherDetails:{$slice: 1}});这将产生以下输出-
{ "_id" : ObjectId("5cdfca52bf3115999ed51205"), "UserId" : 101, "UserName" : "Carol", "UserOtherDetails" : [ { "UserFriendName" : "Sam" } ] }