$each modifier with $push in MongoDB - Step-by-Step Execution

Concept Flow - $each modifier with $push

Start with document

↓

$push with $each detected

↓

Iterate over each element in array

↓

Add element to target array field

↓

Update document with new array

↓

End with updated document

This flow shows how MongoDB uses $push with $each to add multiple elements to an array field one by one.

Execution Sample

MongoDB

db.collection.updateOne(
  { _id: 1 },
  { $push: { scores: { $each: [85, 90, 95] } } }
)

Adds multiple scores (85, 90, 95) to the 'scores' array in the document with _id 1.

Execution Table

Step	Action	Current 'scores' array	Element Added	Resulting 'scores' array
1	Start update	[80, 82]	N/A	[80, 82]
2	Push 85	[80, 82]	85	[80, 82, 85]
3	Push 90	[80, 82, 85]	90	[80, 82, 85, 90]
4	Push 95	[80, 82, 85, 90]	95	[80, 82, 85, 90, 95]
5	Update complete	[80, 82, 85, 90, 95]	N/A	[80, 82, 85, 90, 95]

💡 All elements in $each array pushed; update operation finished.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
scores	[80, 82]	[80, 82, 85]	[80, 82, 85, 90]	[80, 82, 85, 90, 95]	[80, 82, 85, 90, 95]

Key Moments - 2 Insights

Why does $push need $each to add multiple elements?

Without $each, $push adds only one element. The execution_table shows each element added step-by-step using $each (rows 2-4).

Does $push with $each replace the array or add to it?

It adds elements to the existing array, not replace it. See how 'scores' grows after each step in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'scores' array after step 3?

A [80, 82, 85]

B [80, 82, 85, 90]

C [85, 90]

D [80, 82]

💡 Hint

Check the 'Resulting scores array' column at step 3 in execution_table.

At which step does the last element get added to the array?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Element Added' column in execution_table to find when 95 is added.

If we remove $each and try to push [85, 90, 95] directly, what happens?

A Each element is added separately automatically

B Update fails with error

C The entire array [85, 90, 95] is added as one element

D Nothing changes in the array

💡 Hint

Recall that $push without $each adds one element; here that element would be the whole array.

Concept Snapshot

$push with $each lets you add multiple elements to an array field.
Syntax: {$push: {field: {$each: [val1, val2, ...]}}}
Without $each, $push adds only one element.
Each element is appended in order.
Useful for batch updates to arrays.

Full Transcript

This visual execution shows how MongoDB's $push operator combined with $each modifier adds multiple elements to an array field in a document. Starting with an array [80, 82], each element from [85, 90, 95] is added one by one, updating the array step-by-step until it becomes [80, 82, 85, 90, 95]. The $each modifier is necessary to push multiple elements; without it, $push would add the entire array as a single element. The execution table and variable tracker clearly show the array's state after each addition, helping beginners understand the incremental update process.

Practice

1. What does the $each modifier do when used with $push in MongoDB?

easy

A. It sorts the array elements after pushing a new value.

B. It adds multiple values to an array in a single update operation.

C. It replaces the entire array with a new set of values.

D. It removes multiple values from an array.

Solution

1. Step 1: Understand $push usage

   $push adds elements to an array field in a document.

2. Step 2: Role of $each modifier

   $each allows adding multiple elements at once inside $push, making the update efficient.

3. Final Answer:

   It adds multiple values to an array in a single update operation. -> Option B

4. Quick Check:

   $each adds many items [OK]

Hint: Remember: $each adds many items inside $push [OK]

Common Mistakes:

• Thinking $each removes items
• Using $each outside $push
• Confusing $each with sorting

2. Which of the following is the correct syntax to add multiple values ["a", "b", "c"] to the array field tags using $push with $each?

easy

A. { $push: { tags: { $each: ["a", "b", "c"] } } }

B. { $push: { tags: ["a", "b", "c"] } }

C. { $push: { tags: { $add: ["a", "b", "c"] } } }

D. { $push: { tags: { $each: "a", "b", "c" } } }

Solution

1. Step 1: Check correct $push syntax

   $push requires a field and a value or modifier object.

2. Step 2: Validate $each usage

   $each must be an array inside $push to add multiple items.

3. Final Answer:

   { $push: { tags: { $each: ["a", "b", "c"] } } } -> Option A

4. Quick Check:

   Correct $each syntax = { $push: { tags: { $each: ["a", "b", "c"] } } } [OK]

Hint: Use $each with an array inside $push [OK]

Common Mistakes:

• Passing array directly without $each
• Using invalid operator like $add
• Incorrect $each format without array brackets

3. Given the document { _id: 1, colors: ["red"] }, what will be the colors array after running this update?

{ $push: { colors: { $each: ["blue", "green"] } } }

medium

A. ["red", ["blue", "green"]]

B. ["blue", "green"]

C. ["red", "blue", "green"]

D. ["red"]

Solution

1. Step 1: Initial array and update operation

   Original array is ["red"]. The update uses $push with $each to add "blue" and "green".

2. Step 2: Effect of $each inside $push

   $each adds each element individually, so "blue" and "green" are appended to the array.

3. Final Answer:

   ["red", "blue", "green"] -> Option C

4. Quick Check:

   Array after update = ["red", "blue", "green"] [OK]

Hint: $each adds items individually, not as a nested array [OK]

Common Mistakes:

• Expecting nested array inside colors
• Replacing array instead of pushing
• Ignoring original array elements

4. What is wrong with this update operation?

{ $push: { items: { $each: "apple", "banana" } } }

medium

A. The $each value must be an array, not separate strings.

B. The $push operator cannot be used with $each.

C. The field name items is invalid.

D. The update syntax is correct and will work as expected.

Solution

1. Step 1: Analyze $each value type

   $each requires an array of values, but here it is given as separate strings without array brackets.

2. Step 2: Identify syntax error

   Passing multiple strings without array brackets causes a syntax error in MongoDB update.

3. Final Answer:

   The $each value must be an array, not separate strings. -> Option A

4. Quick Check:

   $each needs array brackets [OK]

Hint: Always wrap $each values in square brackets [OK]

Common Mistakes:

• Passing multiple values without array brackets
• Confusing $each with $addToSet
• Assuming $push alone can add multiple items

5. You want to add multiple new tags ["urgent", "review", "backend"] to the tags array of a document, but only if they are not already present. Which update operation correctly uses $push with $each and ensures no duplicates?

hard

A. { $push: { tags: { $each: ["urgent", "review", "backend"] } } }

B. { $addToSet: { tags: ["urgent", "review", "backend"] } }

C. { $push: { tags: ["urgent", "review", "backend"] } }

D. { $addToSet: { tags: { $each: ["urgent", "review", "backend"] } } }

Solution

1. Step 1: Understand requirement to avoid duplicates

   $push adds items regardless of duplicates, so it doesn't fit the requirement.

2. Step 2: Use $addToSet with $each

   $addToSet adds unique values only, and supports $each to add multiple items without duplicates.

3. Final Answer:

   { $addToSet: { tags: { $each: ["urgent", "review", "backend"] } } } -> Option D

4. Quick Check:

   $addToSet + $each avoids duplicates [OK]

Hint: Use $addToSet with $each to add unique items [OK]

Common Mistakes:

• Using $push which allows duplicates
• Not using $each with $addToSet
• Passing array directly without $each