MongoDB $push Operator

MongoDB $push operator appends a value to the already existing array within a document.

The $push operator in MongoDB is used to update the array in MongoDB. It inserts a new value into the array in the specific index.

If the mentioned field is absent in the document to update, the $push operator adds it as a new field and includes the mentioned value as its element.

If the value is an array, the $push operator appends the whole array as a single element. To add each element of the value separately, the $push operator can be used with the $each modifier.

The syntax for using the $push operator is:

{ $push: { <field1>: <value1>, ... } }

Here, <field> can specify with dot notation in embedded/nested documents or an array.

We can also use the modifiers with the $push operator:

Syntax:

{ $push: { <field1>: { <modifier1>: <value1>, ... }, ... } }

The processing of the push operation with modifiers works in the following order:

• First update the array to add items in the correct position.
• Second, apply sort if specified.
• Third slice the array if specified.
• Fourth store the array.

Note:

Here the order in which the modifiers appear in the $push operator does not matter.

List of $push operator modifiers

Below are the modifiers in MongoDB that can be used with $push operator.

In the following examples, we are working with

Database: w3wiki
Collection: contributor
Document: two documents that contain the details of the contributor in the form of field-value pairs

Appending a single value to an array example using $push operator

In this example, we are appending a single value, i.e., “C++” to an array field, i.e., language field in the document that satisfy the condition(name: “Rohit”).

Query:

db.contributor.update({name: "Rohit"}, {$push: {language: "C++"}})

Output:

Appending multiple values to an array using $push operator

In this example, we are appending multiple values, i.e., [“C”, “Ruby”, “Go”] to an array field, i.e., language field in the document that satisfy the condition(name: “Sumit”).

Query:

db.contributor.update({name: "Sumit"}, {$push: {language: {$each: ["C", "Ruby", "Go"]}}})

Output:

Appending multiple values to an array in the nested/embedded document using $push operator:

In this example, we are appending multiple values, i.e., [89, 76.4] to an array field, i.e., personal.semesterMarks field of a nested/embedded document.

Query:

db.contributor.update({name: "Sumit"}, 
                      {$push: {"personal.semesterMarks": {$each: [89, 76.4]}}})

Output:

Using modifiers with $push operator example

In this example, we are using multiple modifiers like $each, $sort, and $slice with $push operator.

Query:

db.contributor.update({name: "Rohit"},
                      {$push: { language: { $each: ["C", "Go"],
                                $sort: 1, $slice: 4}}})

Here,

• The $each modifier is used to add multiple documents to the language array.
• The $sort modifier is used to sort all the items of the modified language array in ascending.
• The $slice modifier is used to keep only the first four sorted items of the language array.

Output:

• The $push operator in MongoDB is used append a new value into the array in the specific index.
• If the specified field in the $push operator is not present in the document, then this operator will add the array field with the value as its items.
• You can use this operator with methods like update(), findAndModify(), etc., according to your requirements.
• The $push operator inserts items at the end of the array.
• If the specified field in the $push operator is not an array, then this operation will fail.
• If the value of the $push the operator is an array, then this operator will append the whole array as a single element. And if you want to add each item of the value separately, then you can use $each modifier with $push the operator.