如何在 MongoDB 嵌入式文档中查找特定元素?
mongodbdatabasebig data analytics
要查找特定元素，请使用 MongoDB 中的 $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")
}
借助 find() 方法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" } ] }
以下是在嵌入文档中查找特定元素的查询 −
> 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" } }
