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