MongoDB – $position Modifier

The $position modifier in MongoDB is a powerful tool used with the $push array update operator to control the exact position where new elements should be inserted within an array. This modifier allows for the precise insertion of elements at a specific index which enables the maintenance of a desired order or structure in the array.

In this article, We will learn about the MongoDB $position Modifier in detail by understanding various examples and so on.

MongoDB $position Modifier

• The $position modifier in MongoDB is used with the $push array update operator to specify the position in the array where the new elements should be inserted.
• It allows precise control over where new elements are inserted in an array which is useful for maintaining a specific order or structure within the array.
• By specifying the index, we can insert elements in a predefined position rather than just appending them to the end of the array.

Syntax:

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

Here, <number> indicates the position of the item in the array according to the zero-based index.

• If the value of <number> (non-negative number corresponds to the array which is starting from the beginning of the array) is greater or equal to the length of the array, then this modifier does not work and the $push operation adds items to the end of the array.
• Starting from MongoDB version 3.6, $position modifier accepts the negative index value. When the value of <number> is negative, then the $position operation counts the item from the last of the array, but does not include the last element.
• For example, if the value of $position modifier is -1, then it indicates the position just before the last item in the array and if you specify multiple items in the $each array, then the last added item is in the specified position from the end. If the value of <number> is greater than or equal to the length of the array, then the $push operation adds items to the beginning of the array.
• $position modifier must appear with $each modifier in the $push operator. If you use the $position modifier without $each modifier, then you will get an error.

Examples of MongoDB $position

In the following examples, we are working with:

Database: GeeksforGeeks

Collection: contributor

Document: two documents that contain the details of the contributor in the form of field-value pairs.

Example 1: Adding items at the start of the array

In this example, we are adding items, i.e., [“C#”, “Perl”] in the beginning(i.e., position 0 )of the language field.

db.contributor.update({name: "Rohit"}, 
                      {$push: { language: { $each: ["C#", "Perl"],
                        $position: 0}}})

Output:

Example 2: Adding items to the middle of the array

In this example, we are adding items, i.e., [“Perl”] in the middle(i.e., position 2 )of the language field.

db.contributor.update({name: "Suman"}, 
                      {$push: { language: { $each: [ "Perl"], 
                        $position: 2}}})

Output:

Example 3: Using a negative index to add items to the array

In this example, we are adding items, i.e., [“C”] just before the last item in the language field.

db.contributor.update({name: "Rohit"},
                      {$push: { language: { $each: [ "C"], 
                        $position: -1}}})

Output:

Conclusion

Overall, $position modifier enhances the functionality of array updates in MongoDB by allowing for the precise placement of new elements within an array. This capability is particularly useful for maintaining the order of elements and ensuring that data remains structured according to specific requirements

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Article Tags :

• Databases
• MongoDB
• MongoDB
• MongoDB-modifier