$each modifier with $push in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after using $push with $each?

Consider a MongoDB collection students with a document:

{ "_id": 1, "name": "Alice", "scores": [85, 90] }

We run this update:

db.students.updateOne({ _id: 1 }, { $push: { scores: { $each: [95, 100] } } })

What will be the scores array after this update?

A [85, 90]

B [95, 100]

C [85, 90, [95, 100]]

D [85, 90, 95, 100]

💡 Hint

The $each modifier adds multiple values individually to the array.

The $push operator adds elements to an array. When combined with $each, it adds each element from the given array separately. So the original array [85, 90] gets 95 and 100 appended individually, resulting in [85, 90, 95, 100].

🧠 Conceptual

intermediate

Why use $each with $push instead of pushing an array directly?

In MongoDB, what is the main reason to use $each with $push when adding multiple elements to an array?

A To sort the array after pushing

B To replace the entire array with the new array

C To add each element individually instead of nesting the array inside the array

D To remove duplicate elements automatically

💡 Hint

Think about what happens if you push an array without $each.

Using $push with $each adds each element of the array separately. Without $each, pushing an array adds the whole array as a single nested element, which is usually not desired.

📝 Syntax

advanced

Identify the correct syntax for using $push with $each and $slice

Which of the following MongoDB update statements correctly uses $push with $each and $slice to add elements and keep only the last 3 elements in the array?

A db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $slice: -3 } } })

B db.collection.updateOne({}, { $push: { items: { $slice: -3, $each: [1,2,3] } } })

C db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $slice: 3 } } })

D db.collection.updateOne({}, { $push: { items: { $each: [1,2,3], $sort: 1, $slice: 3 } } })

💡 Hint

The order of modifiers inside $push matters and $slice uses positive or negative integers.

The correct syntax places $each first, then $slice. Using $slice: -3 keeps the last 3 elements. Option A uses $slice: -3 which keeps the last 3 elements after pushing.

🔧 Debug

advanced

Why does this $push with $each update fail?

Given this update command:

db.products.updateOne({ _id: 10 }, { $push: { tags: { $each: "new", $position: 0 } } })

Why does this update fail?

A Because $each requires an array, but "new" is a string

B Because $position cannot be used with $each

C Because $push cannot add elements at position 0

D Because the document with _id 10 does not exist

💡 Hint

Check the data type expected by $each.

$each expects an array of elements to add. Passing a string "new" causes a type error because a string is not an array.

❓ optimization

expert

Optimizing multiple $push operations with $each

You want to add multiple tags to a document's tags array in MongoDB. Which update operation is the most efficient and atomic way to add "red", "blue", and "green" tags at once?

A Replace the entire tags array with ["red", "blue", "green"] using $set

B Use { $push: { tags: { $each: ["red", "blue", "green"] } } } in one update

C Use $addToSet three times in one update to add each tag

D Run three separate updates each with $push: { tags: "red" }, then "blue", then "green"

💡 Hint

Think about atomicity and minimizing database operations.

Using $push with $each adds multiple elements in one atomic update, which is efficient and avoids multiple round-trips. Running multiple updates or replacing the array is less efficient or destructive.

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