获取数组中的第一个元素，然后使用MongoDB Aggregate返回？
将$unwind运算符与$project一起使用，以获取数组中的第一个元素。让我们创建包含文档的集合。以下是查询
>db.getFirstElementInArrayDemo.insertOne({"StudentName":"John","StudentSubject":["MongoDB","Python","MySQL"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9c41292d6669774125244e")
}
>db.getFirstElementInArrayDemo.insertOne({"StudentName":"Chris","StudentSubject":["Java","C"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9c413f2d6669774125244f")
}
>db.getFirstElementInArrayDemo.insertOne({"StudentName":"Robert","StudentSubject":["C++","Ruby"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9c41532d66697741252450")
}以下是在find()方法的帮助下显示集合中所有文档的查询
> db.getFirstElementInArrayDemo.find().pretty();
这将产生以下输出
{
"_id" : ObjectId("5c9c41292d6669774125244e"),
"StudentName" : "John",
"StudentSubject" : [
"MongoDB",
"Python",
"MySQL"
]
}
{
"_id" : ObjectId("5c9c413f2d6669774125244f"),
"StudentName" : "Chris",
"StudentSubject" : [
"Java",
"C"
]
}
{
"_id" : ObjectId("5c9c41532d66697741252450"),
"StudentName" : "Robert",
"StudentSubject" : [
"C++",
"Ruby"
]
}以下是获取数组中的第一个元素并使用聚合返回的查询
> db.getFirstElementInArrayDemo.aggregate([
... {$unwind:"$StudentSubject"},
... {$group:{"_id":"$_id","FirstElement":{$first:"$StudentSubject"}}},
... {$project:{"_id":0,"FirstElement":1}}
... ]);这将产生以下输出
{ "FirstElement" : "Java" }
{ "FirstElement" : "C++" }
{ "FirstElement" : "MongoDB" }您可以使用$slice运算符find()来获取数组中的第一个元素。以下是查询
> db.getFirstElementInArrayDemo.find({},{"StudentSubject":{$slice:1}});这将产生以下输出
{ "_id" : ObjectId("5c9c41292d6669774125244e"), "StudentName" : "John", "StudentSubject" : [ "MongoDB" ] }
{ "_id" : ObjectId("5c9c413f2d6669774125244f"), "StudentName" : "Chris", "StudentSubject" : [ "Java" ] }
{ "_id" : ObjectId("5c9c41532d66697741252450"), "StudentName" : "Robert", "StudentSubject" : [ "C++" ] }