$position¶

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Definition
Examples

Definition¶

$position¶

2.6 新版功能.

The $position modifier specifies the location in the array at which the $push operator insert 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>
    }
  }
}

The <num> is a non-negative number that corresponds to the position in the array, based on a zero-based index.

If the <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.

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 ] }

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