Count the number of items in an array in MongoDB?

To count the number of items in an array, you can use $size operator. The syntax is as follows:

db.yourCollectionName.aggregate({$project:{anyFieldName:{$size:"$yourArrayName"}}}).prett
y();

To understand the above syntax, let us create a collection with document. The query to create a collection with document is as follows:

>db.getSizeOfArray.insertOne({"StudentId":1,"StudentName":"Larry","StudentMarks":[87,34,5
6,77,89,90]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c6ebc536fd07954a4890680")
}
>db.getSizeOfArray.insertOne({"StudentId":2,"StudentName":"Sam","StudentMarks":[90,76,56
]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c6ebc6b6fd07954a4890681")
}
>db.getSizeOfArray.insertOne({"StudentId":3,"StudentName":"Carol","StudentMarks":[90,76]})
;
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c6ebc7a6fd07954a4890682")
}

Now you can display all documents from a collection with the help of find() method. The query is as follows:

> db.getSizeOfArray.find().pretty();

The following is the output:

{
   "_id" : ObjectId("5c6ebc536fd07954a4890680"),
   "StudentId" : 1,
   "StudentName" : "Larry",
   "StudentMarks" : [
      87,
      34,
      56,
      77,
      89,
      90
   ]
}
{
   "_id" : ObjectId("5c6ebc6b6fd07954a4890681"),
   "StudentId" : 2,
   "StudentName" : "Sam",
   "StudentMarks" : [
      90,
      76,
      56
   ]
}
{
   "_id" : ObjectId("5c6ebc7a6fd07954a4890682"),
   "StudentId" : 3,
   "StudentName" : "Carol",
   "StudentMarks" : [
      90,
      76
   ]
}

The following is the query to count the number of items in an array:

>db.getSizeOfArray.aggregate({$project:{NumberOfItemsInArray:{$size:"$StudentMarks"}}}).p
retty();

The following is the output:

{ "_id" : ObjectId("5c6ebc536fd07954a4890680"), "NumberOfItemsInArray" : 6 }
{ "_id" : ObjectId("5c6ebc6b6fd07954a4890681"), "NumberOfItemsInArray" : 3 }
{ "_id" : ObjectId("5c6ebc7a6fd07954a4890682"), "NumberOfItemsInArray" : 2 }