$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

Need a 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

Need a 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

Need a 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

Need a hint?

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