MongoDB $size Operator

What is the $size operator in MongoDB?

MongoDB provides a variety of array expression operators, and the $size operator is one of those operators. The $size operator is used to find the total number of elements present in the given array, and it is also used in the aggregation pipeline stages.

Syntax of the $size operator:

Important points:

If the argument is missing, then it gives the error.
If the argument does not resolve to an array, then it gives the error.

Examples:

In the following examples, we are working with:

Database: JavaTpoint
Collection: students
Document: Six documents that contain the details of the students

>db.students.find().pretty()
{
   "_id" : 1,
   "name" : "Jonny",
   "class" : "X",
   "rollNo" : 401,
   "age" : 18,
   "marks" : [ 55, 60, 70, 45, 95, 68 ],
   "extraMarks" : {
                    "practical" : [ 21, 18, 25, 30 ],
                    "attendance" : [ 5, 9 ]
                  }
   "gender" : "Male",
   "bloodgroup" : "A+"
}
{
   "_id" : 2,
   "name" : "Carry",
   "class" : "IX",
   "rollNo" : 35,
   "age" : 17,
   "marks" : [ 85, 40, 90, 75, 85, 77 ],
   "gender" : "Male",
   "bloodgroup" : "B+"
}
{
   "_id" : 3,
   "name" : "Jin",
   "class" : "IX",
   "rollNo" : 49,
   "age" : 17,
   "marks" : [ 85, 70, 80, 95, 94, 81 ],
   "gender" : "Female",
   "bloodgroup" : "O+"
}
{
   "_id" : 4,
   "name" : "Thomas",
   "class" : "X",
   "rollNo" : 61,
   "age" : 18,
   "marks" : [ 91, 65, 71, 63, 98, 76 ],
   "extraMarks" : {
                    "practical" : [ 26, 28, 25, 29 ],
                    "attendance" : [ 8, 8 ]
                  }
   "gender" : "Male",
   "bloodgroup" : "A+"
}
{
   "_id" : 5,
   "name" : "Mia",
   "class" : "IX",
   "rollNo" : 308,
   "age" : 17,
   "marks" : [ 97, 98, 95, 98 ],
   "gender" : "Female",
   "bloodgroup" : "B+"
}
{
   "_id" : 6,
   "name" : "Oats",
   "class" : "IX",
   "rollNo" : 75,
   "age" : 18,
   "marks" : [ 99, 98, 98, 95, 96 ],
   "gender" : "Male",
   "bloodgroup" : "A+"
}

Example 1: Using $size operator

In this example, we are going to find the total size of the "marks" field using the $size operator.

db.students.aggregate(
   [
     { $match : { class : "IX" }},
     {
       $project:
          {
            _id : 0,
            name : 1,
            class : 1,
            rollNo : 1,
            marks : 1,
            gender : 1,
            markssize : { $size: "$marks" }
          }
     }
   ]
)

Output:

{
   "name" : "Carry",
   "class" : "IX",
   "rollNo" : 35,
   "marks" : [ 85, 40, 90, 75, 85, 77 ],
   "gender" : "Male",
   "markssize" : 6
}
{
   "name" : "Jin",
   "class" : "IX",
   "rollNo" : 49,
   "marks" : [ 85, 70, 80, 95, 94, 81 ],
   "gender" : "Female",
   "markssize" : 6
}
{
   "name" : "Mia",
   "class" : "IX",
   "rollNo" : 308,
   "marks" : [ 97, 98, 95, 98 ],
   "gender" : "Female",
   "markssize" : 4
}
{
   "name" : "Oats",
   "class" : "IX",
   "rollNo" : 75,
   "marks" : [ 99, 98, 98, 95, 96 ],
   "gender" : "Male",
   "markssize" : 5
}

Example 2: Using $size operator in the embedded document

In this example, we are going to find the total size of the extraMarks.practical field using the $size operator.

db.students.aggregate(
   [
     { $match : { class : "X" }},
     {
       $project:
          {
            _id : 0,
            name : 1,
            class : 1,
            rollNo : 1,
            marks : 1,
            extraMarks : 1
            gender : 1,
            result : { $size: "$extraMarks.practical" }
          }
     }
   ]
)

Output:

{
   "name" : "Jonny",
   "class" : "X",
   "rollNo" : 401,
   "marks" : [ 55, 60, 70, 45, 95, 68 ],
   "extraMarks" : {
                    "practical" : [ 21, 18, 25, 30 ],
                    "attendance" : [ 5, 9 ]
                  }
   "gender" : "Male",
   "result" : 4
}
{
   "name" : "Thomas",
   "class" : "X",
   "rollNo" : 61,
   "marks" : [ 91, 65, 71, 63, 98, 76 ],
   "extraMarks" : {
                    "practical" : [ 26, 28, 25, 29 ],
                    "attendance" : [ 8, 8 ]
                  }
   "gender" : "Male",
   "result" : 4
}

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