$push operator for adding to arrays in MongoDB - Step-by-Step Execution

Concept Flow - $push operator for adding to arrays

Start with document

↓

Locate array field

↓

$push operator adds element

↓

Array grows by one element

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Save updated document

The $push operator finds the array field in a document and adds a new element to the end of that array, then saves the updated document.

Execution Sample

MongoDB

db.users.updateOne(
  { name: "Alice" },
  { $push: { hobbies: "reading" } }
)

This command adds the string "reading" to the hobbies array of the user named Alice.

Execution Table

Step	Action	Document Before	Array Field Before	Element Added	Array Field After	Document After
1	Find document with name 'Alice'	{ name: 'Alice', hobbies: ['swimming', 'cycling'] }	['swimming', 'cycling']		['swimming', 'cycling']	{ name: 'Alice', hobbies: ['swimming', 'cycling'] }
2	Apply $push to hobbies array	{ name: 'Alice', hobbies: ['swimming', 'cycling'] }	['swimming', 'cycling']	"reading"	['swimming', 'cycling', 'reading']	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }
3	Save updated document	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }	['swimming', 'cycling', 'reading']		['swimming', 'cycling', 'reading']	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }
4	Exit					Update complete

💡 Update completes after adding element to array and saving document

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Document	{ name: 'Alice', hobbies: ['swimming', 'cycling'] }	{ name: 'Alice', hobbies: ['swimming', 'cycling'] }	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }	{ name: 'Alice', hobbies: ['swimming', 'cycling', 'reading'] }
hobbies array	['swimming', 'cycling']	['swimming', 'cycling']	['swimming', 'cycling', 'reading']	['swimming', 'cycling', 'reading']	['swimming', 'cycling', 'reading']

Key Moments - 3 Insights

What happens if the array field does not exist in the document?

If the array field does not exist, $push creates the array field and adds the element as the first item. This is not shown in the table but is important to know.

Does $push add the element to the beginning or end of the array?

$push always adds the element to the end of the array, as shown in step 2 where "reading" is added after existing elements.

What if the field is not an array but a different type?

If the field is not an array, $push will cause an error because it expects an array to add elements to.

Visual Quiz - 3 Questions

Look at the execution table, what is the hobbies array after step 2?

A ['swimming', 'cycling']

B ['swimming', 'cycling', 'reading']

C ['reading', 'swimming', 'cycling']

D []

💡 Hint

Check the 'Array Field After' column in row with Step 2

At which step is the new element actually added to the array?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Element Added' and 'Array Field After' columns in the execution table

If the hobbies field did not exist before, what would $push do?

A Do nothing

B Throw an error

C Create hobbies as an array with the new element

D Replace the document

💡 Hint

Refer to key moments about missing array field behavior

Concept Snapshot

$push operator adds an element to the end of an array field in a MongoDB document.
Syntax: { $push: { arrayField: newElement } }
If the array field doesn't exist, it creates it.
If the field is not an array, it causes an error.
Used in update commands like updateOne or updateMany.

Full Transcript

The $push operator in MongoDB updates a document by adding a new element to the end of an existing array field. The process starts by finding the document, then locating the array field. The new element is added to the end of this array, and the updated document is saved. If the array field does not exist, $push creates it with the new element as the first item. If the field is not an array, an error occurs. This operator is useful for growing arrays inside documents without replacing the whole array.