$slice modifier with $push in MongoDB - Mini Project: Build & Apply

Manage Recent Comments with $slice Modifier and $push in MongoDB

📖 Scenario: You are managing a blog database where each post has a list of comments. To keep the comments list manageable, you want to store only the 3 most recent comments per post.

🎯 Goal: Build a MongoDB update operation that adds a new comment to a post's comments array and uses the $slice modifier with $push to keep only the latest 3 comments.

📋 What You'll Learn

Create a document for a blog post with an empty comments array.

Define a new comment object with specific fields.

Use $push with $each and $slice to add the new comment and keep only 3 comments.

Complete the update operation to apply this change to the post.

💡 Why This Matters

🌍 Real World

Blog platforms and social media apps often limit the number of stored comments or messages to improve performance and user experience.

💼 Career

Understanding how to use MongoDB update operators like $push with $slice is important for backend developers managing dynamic array data efficiently.

1

Create the initial blog post document

Create a MongoDB document called post with _id set to 1 and an empty array field called comments.

MongoDB

# Create the post document with _id 1 and empty comments array
# Your code here

Hint

Use a dictionary with keys "_id" and "comments".

2

Define a new comment object

Create a variable called new_comment that is a dictionary with user set to "Alice" and text set to "Great post!".

MongoDB

post = {"_id": 1, "comments": []}
# Define the new_comment dictionary
# Your code here

Hint

Use a dictionary with keys "user" and "text" exactly as shown.

3

Create the update operation with $push and $slice

Create a variable called update_operation that uses $push on the comments field with $each containing new_comment and $slice set to -3 to keep only the last 3 comments.

MongoDB

post = {"_id": 1, "comments": []}
new_comment = {"user": "Alice", "text": "Great post!"}
# Create the update_operation using $push with $each and $slice
# Your code here

Hint

Use $push with $each as a list containing new_comment and $slice set to -3.

4

Complete the update command to apply the operation

Create a variable called update_command that uses db.posts.update_one to update the document with _id 1 by applying update_operation.

MongoDB

post = {"_id": 1, "comments": []}
new_comment = {"user": "Alice", "text": "Great post!"}
update_operation = {
    "$push": {
        "comments": {
            "$each": [new_comment],
            "$slice": -3
        }
    }
}
# Create the update_command to update the post with _id 1 using update_operation
# Your code here

Hint

Use db.posts.update_one with a filter {"_id": 1} and the update_operation variable.

Practice

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

easy

A. Duplicates the array elements before pushing new ones

B. Removes all elements from the array before adding new ones

C. Limits the size of the array after adding new elements

D. Sorts the array elements in ascending order

Solution

1. Step 1: Understand $push behavior

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

2. Step 2: Role of $slice modifier

   $slice limits the array size after pushing, keeping only a specified number of elements.

3. Final Answer:

   Limits the size of the array after adding new elements -> Option C

4. Quick Check:

   $slice limits array size [OK]

Hint: Remember: $slice trims array size after $push [OK]

Common Mistakes:

• Thinking $slice removes elements before pushing
• Confusing $slice with sorting or duplicating
• Assuming $slice adds elements instead of limiting

2. Which of the following is the correct syntax to push multiple items and limit the array size to 3 using $slice in MongoDB?

easy

A. { $push: { scores: { $each: [90, 85], $slice: 3 } } }

B. { $push: { scores: { $slice: [3], $each: [90, 85] } } }

C. { $push: { scores: { $each: 3, $slice: [90, 85] } } }

D. { $push: { scores: { $slice: [90, 85], $each: 3 } } }

Solution

1. Step 1: Identify correct order of modifiers

   The $each modifier must come before $slice inside $push.

2. Step 2: Validate syntax correctness

   { $push: { scores: { $each: [90, 85], $slice: 3 } } } correctly uses $each: [90, 85] followed by $slice: 3 to limit array size.

3. Final Answer:

   { $push: { scores: { $each: [90, 85], $slice: 3 } } } -> Option A

4. Quick Check:

   $each before $slice [OK]

Hint: Put $each before $slice inside $push [OK]

Common Mistakes:

• Swapping order of $each and $slice
• Using arrays instead of numbers for $slice
• Placing $slice outside $push

3. Given the document { _id: 1, items: [1, 2, 3, 4] }, what will be the items array after running this update?
db.collection.updateOne({ _id: 1 }, { $push: { items: { $each: [5, 6], $slice: -4 } } })

medium

A. [3, 4, 5, 6]

B. [1, 2, 3, 4, 5, 6]

C. [2, 3, 4, 5]

D. [5, 6]

Solution

1. Step 1: Understand $each and $slice with negative value

   $each adds [5, 6] to the array. $slice: -4 keeps the last 4 elements of the array.

2. Step 2: Calculate new array content

   Original array: [1, 2, 3, 4]
   After push: [1, 2, 3, 4, 5, 6]
   Apply $slice: -4: keep last 4 elements -> [3, 4, 5, 6]

3. Final Answer:

   [3, 4, 5, 6] -> Option A

4. Quick Check:

   Negative $slice keeps last elements [OK]

Hint: Negative $slice keeps last N elements [OK]

Common Mistakes:

• Ignoring $slice and keeping all elements
• Using positive $slice logic incorrectly
• Removing new elements instead of old

4. You run this update:
db.collection.updateOne({ _id: 2 }, { $push: { logs: { $each: ["a", "b"], $slice: 1 } } })
But the array logs ends up with 1 element that is an old one instead of a newly pushed one. What is the likely mistake?

medium

A. Not specifying $position causes elements to append incorrectly

B. Missing $each causes syntax error

C. Using $slice with a negative number causes array to clear

D. Using positive $slice keeps first elements instead of newest ones

Solution

1. Step 1: Understand positive $slice behavior

   Positive $slice: 1 keeps only the first element of the array after push.

2. Step 2: Why old elements remain

   Positive $slice: 1 keeps the first (oldest) element of the resulting array since new elements are appended to the end.

3. Step 3: Common confusion

   Positive $slice keeps elements from the start, so if you want to keep the latest elements, use negative $slice.

4. Final Answer:

   Using positive $slice keeps first elements instead of newest ones -> Option D

5. Quick Check:

   Positive $slice keeps start elements [OK]

Hint: Use negative $slice to keep newest elements [OK]

Common Mistakes:

• Confusing positive and negative $slice behavior
• Omitting $each inside $push
• Expecting $slice to remove elements before push

5. You want to keep only the 5 most recent messages in a chat array after adding new messages ["msg4", "msg5"]. The current array is ["msg1", "msg2", "msg3", "msg4"]. Which update query correctly achieves this?

hard

A. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 5 } } })

B. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } })

C. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 2 } } })

D. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -2 } } })

Solution

1. Step 1: Understand the goal

   We want to add two new messages and keep only the 5 most recent messages in the array.

2. Step 2: Use $each and negative $slice

   $each adds multiple items. Negative $slice: -5 keeps the last 5 elements, which are the most recent.

3. Step 3: Check options

   db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } }) uses $slice: -5 correctly. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 5 } } }) keeps first 5 elements (oldest), C keeps only 2 elements, D keeps last 2 elements, which is too few.

4. Final Answer:

   db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } }) -> Option B

5. Quick Check:

   Negative $slice keeps newest 5 elements [OK]

Hint: Use negative $slice with $each to keep recent items [OK]

Common Mistakes:

• Using positive $slice which keeps oldest items
• Setting $slice too small and losing messages
• Forgetting $each when adding multiple items