MongoDB Array Update Operator - $addToSet

Description

The $addToSet operator adds or appends a value to an array, only if the value does not exist in the array. The $addToSet returns the same array without modifying when the value already is in the array.

The $addToSet operator ensures that there will be no duplicate items in an array at the time of updating, but one thing can be happen that, after adding a value, the order of the elements of an array can be changed.

Syntax:

db.collection.update( { <field>: <value> }, { $addToSet: { <field>: <addition> } } );

Parameters:

Sample collection "student"

 { "_id" : 1, "sem" : 1, "achieve" : [  70,  87,  90 ] }

Example of MongoDB $addToSet operator to append a value

If we want to append a value 92 to the array achieve the following mongodb command can be used -

db.student.update( { "sem": 1}, { $addToSet: { "achieve": 92 } } );

The above example will append a value 92 in the array field achieve.

To see the newly updated document -

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

Output of the command

{ "_id" : 1, "achieve" : [ 70, 87, 90, 92 ], "sem" : 1 }

Example of MongoDB $addToSet operator to add multiple value using $each modifier

If we want to append the values 10 and 11 to the array field achieve, the following mongodb command can be used.

> db.student.update( { "sem": 1}, { $addToSet: { "achieve":{$each:[10,11]}}});

In the above example, using of $each modifier the values 10 and 11 have been appended to the array field achieve.

To see the newly updated document -

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

Output of the command:

{
        "_id" : 1,
        "achieve" : [
                70,
                87,
                90,
                92,
                10,
                11
        ],
        "sem" : 1
}

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