Query array of subdocuments in MongoDB

To query an array of subdocuments, use $unwind in MongoDB. Let us create a collection with documents −

> db.demo499.insertOne({ ... "details": ...    [ ...       { ...          Name :"MIT", ...          Rank: 1, ...          "CountryName":"US" ...       }, ... ...       { ...          Name :"Stanford University", ...          Rank: 2 ...       }, ... ...       { ...          Name :"University of California, Berkeley", ...          Rank: 3 ...       } ...    ] ... } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e87433d987b6e0e9d18f558") }

Display all documents from a collection with the help of find() method −

> db.demo499.find();

This will produce the following output −

{ "_id" : ObjectId("5e87433d987b6e0e9d18f558"), "details" : [ { "Name" : "MIT", "Rank" : 1, "CountryName" : "US" }, { "Name" : "Stanford University", "Rank" : 2 }, { "Name" : "University of California, Berkeley", "Rank" : 3 } ] }

Following is how to query an array of subdocuments −

> db.demo499.aggregate({$unwind: "$details"}, ... {$match: {"details.CountryName":{$exists: true}}}, ... {$project: {"details.Name": 1, "details.CountryName": 1, "_id": 0}})

This will produce the following output −

{ "details" : { "Name" : "MIT", "CountryName" : "US" } }