如何在 MongoDB 嵌入式文档中查找某个元素?
为了查找某个元素,在 MongoDB 中使用 $project。让我们创建一个文档集合 -
> 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") }
使用 find() 方法从集合中显示所有文档 -
> db.demo744.find();
这将生成以下输出 -
{ "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : [ { "studentName" : "Robert", "grade" : "A" }, { "studentName" : "Bob", "grade" : "C" }, { "studentName" : "John", "grade" : "B" }, { "studentName" : "Sam", "grade" : "A" } ] }
以下是查询,用于在嵌入式文档中查找某个元素 -
> db.demo744.aggregate( ... { $unwind: '$studentInformation' }, ... { $match: {'studentInformation.grade':"A"}}, ... { $project: {"studentInformation.studentName": 1}} ... )
这将生成以下输出 -
{ "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : { "studentName" : "Robert" } } { "_id" : ObjectId("5ead928a57bb72a10bcf0684"), "studentInformation" : { "studentName" : "Sam" } }
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