$each modifier with $push in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $push operator do in MongoDB?

The $push operator adds a value to an array field in a MongoDB document. If the field does not exist, it creates the array with the value.

beginner

What is the purpose of the $each modifier when used with $push?

$each allows you to add multiple values to an array at once instead of pushing one value at a time.

beginner

How would you add multiple items ["apple", "banana", "cherry"] to an array field fruits using $push and $each?

{ $push: { fruits: { $each: ["apple", "banana", "cherry"] } } }

intermediate

Can $each be combined with other modifiers like $sort or $slice?

Yes, $each can be combined with $sort to sort the array after pushing, and $slice to limit the array size.

beginner

What happens if you use $push without $each to add multiple values?

Without $each, $push treats the entire list as a single element and adds it as one item to the array.

What does $each do when used with $push in MongoDB?

A Adds multiple values to an array at once

B Removes multiple values from an array

C Sorts the array elements

D Creates a new collection

$each allows adding multiple values to an array in one operation with $push.

Which of the following is the correct syntax to add multiple items to an array field tags?

A { $addToSet: { tags: ["red", "blue"] } }

B { $push: { tags: ["red", "blue"] } }

C { $push: { tags: { $add: ["red", "blue"] } } }

D { $push: { tags: { $each: ["red", "blue"] } } }

Using $each inside $push is the correct way to add multiple values.

What happens if you push an array without $each?

A The entire array is added as one element

B Each element is added separately

C The operation fails

D The array is ignored

Without $each, the array is treated as a single element and pushed as is.

Can $each be used with $sort inside $push?

A Only with <code>$slice</code>, not <code>$sort</code>

B No, they cannot be combined

C Yes, to sort the array after pushing

D Only in aggregation pipelines

$each can be combined with $sort to sort the array after adding new elements.

Which operator creates an array field if it does not exist when pushing values?

A $addToSet

B $push

C $set

D $unset

$push creates the array field if it does not exist before adding values.

Explain how to add multiple items to an array field in MongoDB using $push and $each.

Think about pushing many items at once instead of one by one.

Describe what happens if you use $push without $each to add multiple values.

Consider how MongoDB treats the value given to $push.

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