如何从MongoDB数组中获取特定元素？
您可以使用聚合框架从MongoDB数组中获取特定元素。为了理解这个概念，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.getParticularElement.insertOne({"InstructorName":"Larry","InstructorTechnicalSubject":["Java","C","C++","MongoDB","MySQL","SQL Server"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7ee027559dd2396bcfbfb1")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.getParticularElement.find().pretty();
以下是输出-
{
"_id" : ObjectId("5c7ee027559dd2396bcfbfb1"),
"InstructorName" : "Larry",
"InstructorTechnicalSubject" : [
"Java",
"C",
"C++",
"MongoDB",
"MySQL",
"SQL Server"
]
}这是从数组中获取特定元素的查询-
> db.getParticularElement.aggregate([
... {
... $project:
... {
... ElementFromAnArray: 1,
... FourthElement: { $arrayElemAt: [ "$InstructorTechnicalSubject", 3] },
...
... }
... }
... ]);以下是输出-
{ "_id" : ObjectId("5c7ee027559dd2396bcfbfb1"), "FourthElement" : "MongoDB" }