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$position

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Definition

$position

The $position modifier specifies the location in the array at which the $push operator inserts elements. Without the $position modifier, the $push operator inserts elements to the end of the array. See $push modifiers for more information.

To use the $position modifier, it must appear with the $each modifier.

{
  $push: {
    <field>: {
       $each: [ <value1>, <value2>, ... ],
       $position: <num>
    }
  }
}

Changed in version 3.6: $position can accept a negative array index value to indicate the position starting from the end, counting from (but not including) the last element of the array.

<num> indicates the position in the array, based on a zero-based index:

  • A non-negative number corresponds to the position in the array, starting from the beginning of the array. If the value of <num> is greater or equal to the length of the array, the $position modifier has no effect and $push adds elements to the end of the array.
  • A negative number corresponds to the position in the array, counting from (but not including) the last element of the array. For example, -1 indicates the position just before the last element in the array. If you specify multiple elements in the $each array, the last added element is in the specified position from the end. If the absolute value of <num> is greater than or equal to the length of the array, the $push adds elements to the beginning of the array.

Examples

Add Elements at the Start of the Array

Consider a collection students that contains the following document:

{ "_id" : 1, "scores" : [ 100 ] }

The following operation updates the scores field to add the elements 50, 60 and 70 to the beginning of the array:

db.students.update(
   { _id: 1 },
   {
     $push: {
        scores: {
           $each: [ 50, 60, 70 ],
           $position: 0
        }
     }
   }
)

The operation results in the following updated document:

{ "_id" : 1, "scores" : [  50,  60,  70,  100 ] }

Add Elements to the Middle of the Array

Consider a collection students that contains the following document:

{ "_id" : 1, "scores" : [  50,  60,  70,  100 ] }

The following operation updates the scores field to add the elements 20 and 30 at the array index of 2:

db.students.update(
   { _id: 1 },
   {
     $push: {
        scores: {
           $each: [ 20, 30 ],
           $position: 2
        }
     }
   }
)

The operation results in the following updated document:

{ "_id" : 1, "scores" : [  50,  60,  20,  30,  70,  100 ] }

Use a Negative Index to Add Elements to the Array

Changed in version 3.6: $position can accept a negative array index value to indicate the position starting from the end, counting from (but not including) the last element of the array. For example, -1 indicates the position just before the last element in the array.

Consider a collection students that contains the following document:

{ "_id" : 1, "scores" : [  50,  60,  20,  30,  70,  100 ] }

The following operation specifies -2 for the $position to add 90 at the position two places before the last element, and then 80 at the position two places before the last element.

Important

With a negative index position, if you specify multiple elements in the $each array, the last added element is in the specified position from the end.

db.students.update(
   { _id: 1 },
   {
     $push: {
        scores: {
           $each: [ 90, 80 ],
           $position: -2
        }
     }
   }
)

The operation results in the following updated document:

{ "_id" : 1, "scores" : [ 50, 60, 20, 30, 90, 80, 70, 100 ] }
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