MongoDB查询以查找数组的第一个元素的属性？
您可以为此使用$slice运算符。首先让我们创建一个包含文档的集合-
> db.firstElementOfArray.insertOne(
... {
... _id: 100,
... "Details": [
... {
... "CustomerName": "John",
... "CustomerCountryName":"US"
... }
... ]
... }
... );
{ "acknowledged" : true, "insertedId" : 100 }
> db.firstElementOfArray.insertOne(
... {
... _id: 101,
... "Details": [
... {
... "CustomerName": "Carol",
... "CustomerCountryName":"UK"
... },
... {
... "CustomerName": "David",
... "CustomerCountryName":"AUS"
... }
... ]
... }
... );
{ "acknowledged" : true, "insertedId" : 101 }以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.firstElementOfArray.find().pretty();
这将产生以下输出-
{
"_id" : 100,
"Details" : [
{
"CustomerName" : "John",
"CustomerCountryName" : "US"
}
]
}
{
"_id" : 101,
"Details" : [
{
"CustomerName" : "Carol",
"CustomerCountryName" : "UK"
},
{
"CustomerName" : "David",
"CustomerCountryName" : "AUS"
}
]
}以下是查找数组第一个元素的属性的查询-
> db.firstElementOfArray.find({},{'Details':{$slice:1},'Details.CustomerName':1}).pretty();这将产生以下输出-
{ "_id" : 100, "Details" : [ { "CustomerName" : "John" } ] }
{ "_id" : 101, "Details" : [ { "CustomerName" : "Carol" } ] }