One-to-many embedding pattern in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to embed multiple comments inside a blog post document.

MongoDB

db.posts.insertOne({ title: "My Post", comments: [[1]] })

Drag options to blanks, or click blank then click option'

A{ text: "Great post!" }

B[{ text: "Great post!" }, { text: "Thanks for sharing." }]

C"Great post!", "Thanks for sharing."

D{ text: "Great post!" }, { text: "Thanks for sharing." }

Attempts:

3 left

💡 Hint

Common Mistakes

Putting comments as strings instead of objects.

Wrapping the array of comments in extra brackets.

The comments field is an array of embedded documents. Each comment is an object inside the array.

2fill in blank

medium

Complete the query to find posts that have at least one comment with text 'Nice!'.

MongoDB

db.posts.find({ comments: { $elemMatch: { text: [1] } } })

Drag options to blanks, or click blank then click option'

ANice!

B"Nice!"

C{ $eq: "Nice!" }

D/Nice!/

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string.

Using a regex instead of exact match.

The $elemMatch operator matches array elements. The text field must equal the string "Nice!".

3fill in blank

hard

Fix the error in the update to add a new comment to the post with _id 1.

MongoDB

db.posts.updateOne({ _id: 1 }, { [1]: { comments: { text: "New comment" } } })

Drag options to blanks, or click blank then click option'

A$push

B$add

C$set

D$insert

Attempts:

3 left

💡 Hint

Common Mistakes

Using $set which replaces the whole array instead of adding.

Using non-existent operators like $add or $insert.

The $push operator adds an element to an array field. Here, it adds a new comment object to the comments array.

4fill in blank

hard

Fill both blanks to update the text of the first comment in the post with _id 2.

MongoDB

db.posts.updateOne({ _id: 2 }, { [1]: { "comments.[2].text": "Updated comment" } })

Drag options to blanks, or click blank then click option'

A$set

B$push

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using $push which adds a new comment instead of updating.

Using index 1 which targets the second comment.

Use $set to update a specific field. The first comment is at index 0 in the comments array.

5fill in blank

hard

Fill the blanks to remove comments with text 'Spam' from the post with _id 3.

MongoDB

db.posts.updateOne({ _id: 3 }, { [1]: { comments: { text: [2] } } })

Drag options to blanks, or click blank then click option'

A$pull

B$pop

C"Spam"

D"spam"

Attempts:

3 left

💡 Hint

Common Mistakes

Using $pop which removes only the first or last element.

Using wrong case in the string value.

The $pull operator removes all array elements that match a condition. Here, it removes comments where text equals "Spam".

Practice

1. What is the main advantage of using the one-to-many embedding pattern in MongoDB?

easy

A. It separates related data into different collections for better normalization.

B. It duplicates data across multiple documents to improve write speed.

C. It requires complex joins to retrieve related data.

D. It stores related data inside the main document for faster reads.

Solution

1. Step 1: Understand embedding in MongoDB

   Embedding means storing related data inside the main document instead of separate collections.

2. Step 2: Identify the advantage of embedding

   This approach makes reading related data faster and simpler because all data is in one place.

3. Final Answer:

   It stores related data inside the main document for faster reads. -> Option D

4. Quick Check:

   Embedding = faster reads [OK]

Hint: Embedding means related data inside main document for quick access [OK]

Common Mistakes:

• Confusing embedding with referencing (separate collections)
• Thinking embedding requires joins
• Assuming embedding duplicates data

2. Which of the following is the correct way to embed multiple comments inside a blog post document in MongoDB?

easy

A. { _id: 1, title: 'Post', comments: 'Alice: Nice!, Bob: Great!' }

B. { _id: 1, title: 'Post', comments: { user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' } }

C. { _id: 1, title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] }

D. { _id: 1, title: 'Post', comments: [{ user: 'Alice' }, user: 'Bob', text: 'Great!' ] }

Solution

1. Step 1: Check the structure for embedding multiple comments

   Embedding many comments requires an array of objects inside the main document.

2. Step 2: Identify the correct JSON syntax

   { _id: 1, title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] } correctly uses an array of comment objects inside the comments field.

3. Final Answer:

   { _id: 1, title: 'Post', comments: [{ user: 'Alice', text: 'Nice!' }, { user: 'Bob', text: 'Great!' }] } -> Option C

4. Quick Check:

   Array of objects = correct embedding [OK]

Hint: Embed many items as an array of objects inside the document [OK]

Common Mistakes:

• Using multiple objects without array brackets
• Storing comments as a string instead of objects
• Incorrect JSON syntax with misplaced commas or braces

3. Given the following document in a posts collection:

{ _id: 1, title: 'Hello', comments: [{ user: 'Alice', text: 'Hi' }, { user: 'Bob', text: 'Hello' }] }

What will be the result of the query db.posts.find({ _id: 1 }, { 'comments.user': 1, _id: 0 })?

medium

A. { comments: [{ user: 'Alice' }, { user: 'Bob' }] }

B. { comments: [{ text: 'Hi' }, { text: 'Hello' }] }

C. { _id: 1, comments: [{ user: 'Alice' }, { user: 'Bob' }] }

D. { comments: ['Alice', 'Bob'] }

Solution

1. Step 1: Understand the projection in the query

   The query projects only the comments.user field and excludes _id.

2. Step 2: Determine the output structure

   It returns the comments array with only the user fields inside each comment object.

3. Final Answer:

   { comments: [{ user: 'Alice' }, { user: 'Bob' }] } -> Option A

4. Quick Check:

   Projection of comments.user = array of user objects [OK]

Hint: Projection with dot notation returns array of objects with that field [OK]

Common Mistakes:

• Expecting full comment objects instead of only user fields
• Including _id when excluded in projection
• Getting array of strings instead of objects

4. You have this document:

{ _id: 1, name: 'Store', products: [{ name: 'Pen', price: 1.5 }, { name: 'Pencil', price: 1.0 }] }

Which update query correctly adds a new product { name: 'Eraser', price: 0.5 } to the products array?

medium

A. db.stores.updateOne({ _id: 1 }, { $push: { products: { name: 'Eraser', price: 0.5 } } })

B. db.stores.updateOne({ _id: 1 }, { $addToSet: { products: 'Eraser' } })

C. db.stores.updateOne({ _id: 1 }, { $set: { products: { name: 'Eraser', price: 0.5 } } })

D. db.stores.updateOne({ _id: 1 }, { $push: { products: ['Eraser', 0.5] } })

Solution

1. Step 1: Identify the correct operator to add an object to an array

   The $push operator adds an element to an array field.

2. Step 2: Check the value being pushed

   db.stores.updateOne({ _id: 1 }, { $push: { products: { name: 'Eraser', price: 0.5 } } }) pushes the correct object { name: 'Eraser', price: 0.5 } into the products array.

3. Final Answer:

   db.stores.updateOne({ _id: 1 }, { $push: { products: { name: 'Eraser', price: 0.5 } } }) -> Option A

4. Quick Check:

   $push with object adds to array [OK]

Hint: Use $push with full object to add to embedded array [OK]

Common Mistakes:

• Using $set replaces whole array instead of adding
• Using $addToSet with a string instead of object
• Pushing array instead of object inside array

5. You want to store a user's orders in a single document using one-to-many embedding. Each order has an orderId, date, and a list of items. Which schema design is best to handle up to 100 orders per user efficiently?

hard

A. Separate orders collection with userId reference for each order

B. { _id: userId, name: 'User', orders: [{ orderId, date, items: [{ productId, qty }] }] }

C. { _id: userId, name: 'User', orderIds: [orderId1, orderId2], orders: [] }

D. { _id: userId, name: 'User', orders: { orderId, date, items: [{ productId, qty }] } }

Solution

1. Step 1: Understand embedding for one-to-many with moderate size

   Embedding up to 100 orders inside a user document is reasonable for fast reads.

2. Step 2: Check schema design for correct embedding

   { _id: userId, name: 'User', orders: [{ orderId, date, items: [{ productId, qty }] }] } embeds an array of order objects, each with its own items array, which fits the one-to-many embedding pattern well.

3. Final Answer:

   { _id: userId, name: 'User', orders: [{ orderId, date, items: [{ productId, qty }] }] } -> Option B

4. Quick Check:

   Embed array of orders with nested items for one-to-many [OK]

Hint: Embed arrays of objects for moderate related data size [OK]

Common Mistakes:

• Using object instead of array for multiple orders
• Separating orders into another collection unnecessarily
• Storing only order IDs without details inside user