如何在MongoDB嵌入式文档中找到某个元素
要查找某个元素，请在MongoDB中使用$project。让我们创建一个包含文档的集合-
> db.demo744.insertOne(
... {
... studentInformation:
... [
... {
... studentName:"Robert",
... grade:"A"
... },
... {
... studentName:"Bob",
... grade:"C"
... },
... {
... studentName:"John",
... grade:"B"
... },
... {
... studentName:"Sam",
... grade:"A"
... }
... ]
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5ead928a57bb72a10bcf0684")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo744.find();
这将产生以下输出-
{ "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : [ { "studentName" : "Robert", "grade" : "A" }, { "studentName" : "Bob", "grade" : "C" }, { "studentName" : "John", "grade" : "B" }, { "studentName" : "Sam", "grade" : "A" } ] }以下是在嵌入式文档中查找某个元素的查询-
> db.demo744.aggregate(
... { $unwind: '$studentInformation' },
... { $match: {'studentInformation.grade':"A"}},
... { $project: {"studentInformation.studentName": 1}}
... )这将产生以下输出-
{ "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : { "studentName" : "Robert" } }
{ "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : { "studentName" : "Sam" } }