Native Querying MongoDB inside array and get the count

To query inside array and check for existence to get the count, use $exists. Let us create a collection with documents −

> db.demo296.insertOne( ...   { ...      "id":101, ...      "Name":"Chris", ...      "details":[ ...         { ...            SubjectId:[101,103], ...            "SubjectName":["MySQL","MongoDB"] ...         }, ...         { ...            SubjectId:[102,104], ...            "SubjectName":["Java","C"] ...         } ...      ] ...   } ...); {    "acknowledged" : true,    "insertedId" : ObjectId("5e4d51715d93261e4bc9ea3b") } > db.demo296.insertOne( ...{ ...   "id":102, ...   "Name":"David", ...   "details":[ ...      { ...         SubjectId:[110,113] ... ...      }, ...      { ...         SubjectId:[112,114] ... ...      } ...   ] ...} ...); {    "acknowledged" : true,    "insertedId" : ObjectId("5e4d519b5d93261e4bc9ea3c") } > db.demo296.insertOne( ...   { ...      "id":103, ...      "Name":"Bob", ...      "details":[ ...         { ...            "SubjectName":["C++","Python"] ...         }, ...         { ...            "SubjectName":["Spring","Hibernate"] ...         } ...      ] ...   } ...); {    "acknowledged" : true,    "insertedId" : ObjectId("5e4d52315d93261e4bc9ea3d") }

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

> db.demo296.find();

This will produce the following output −

{    "_id" : ObjectId("5e4d51715d93261e4bc9ea3b"), "id" : 101, "Name" : "Chris", "details" : [       { "SubjectId" : [ 101, 103 ], "SubjectName" : [ "MySQL", "MongoDB" ] },       { "SubjectId" : [ 102, 104 ], "SubjectName" : [ "Java", "C" ] }    ] } {    "_id" : ObjectId("5e4d519b5d93261e4bc9ea3c"), "id" : 102, "Name" : "David", "details" : [       { "SubjectId" : [ 110, 113 ] }, { "SubjectId" : [ 112, 114 ] }    ] } {    "_id" : ObjectId("5e4d52315d93261e4bc9ea3d"), "id" : 103, "Name" : "Bob", "details" : [       { "SubjectName" : [ "C++", "Python" ] }, { "SubjectName" : [ "Spring", "Hibernate" ] }    ] }

Following is how to query MongoDB inside arrays and get the count −

> db.demo296.count( {    'details.SubjectName': {$exists: true }} );

This will produce the following output −

2