Why document design matters in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a document into the collection.

MongoDB

db.users.insertOne({ name: "Alice", age: [1] })

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

Anull

B"25"

Cage

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the number 25 in quotes, making it a string.

Using a variable name instead of a value.

The age field should be a number, so 25 without quotes is correct.

2fill in blank

medium

Complete the code to find documents where the age is greater than 30.

MongoDB

db.users.find({ age: { [1]: 30 } })

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

A$gt

B$lt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt which means less than.

Using $eq which means equal to.

The operator $gt means 'greater than', so it finds ages above 30.

3fill in blank

hard

Fix the error in the update query to set the city field.

MongoDB

db.users.updateOne({ name: "Bob" }, { [1]: { city: "Paris" } })

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

A$update

Bset

C$set

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving out the dollar sign in the operator.

Using a wrong operator like $update.

The correct operator to update fields is $set with a dollar sign.

4fill in blank

hard

Fill both blanks to create a document with an embedded address object.

MongoDB

db.users.insertOne({ name: "Eve", address: { [1]: "123 Main St", [2]: "NY" } })

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

Astreet

Bcity

Cstate

Dzipcode

Attempts:

3 left

💡 Hint

Common Mistakes

Using city instead of state for the second blank.

Using zipcode which is not in the example.

The address has a street and a state field, so use 'street' and 'state'.

5fill in blank

hard

Fill all three blanks to query documents with age greater than 20 and city equals 'Boston'.

MongoDB

db.users.find({ age: { [1]: 20 }, address: { [2]: [3] } })

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

A$gt

Bcity

C"Boston"

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gt for age.

Not putting Boston in quotes.

Using wrong field name instead of city.

Use $gt for age greater than 20, and match city to "Boston".

Practice

1. Why is good document design important in MongoDB?

easy

A. It groups related data together for faster access.

B. It makes the database use more disk space.

C. It forces all data to be stored in separate collections.

D. It prevents any data from being updated.

Solution

1. Step 1: Understand document design purpose

   Good document design groups related data to reduce the number of database lookups.

2. Step 2: Identify the benefit of grouping data

   Grouping related data together makes data access faster and simpler for the application.

3. Final Answer:

   It groups related data together for faster access. -> Option A

4. Quick Check:

   Good design = grouped data = faster access [OK]

Hint: Good design groups related data for speed [OK]

Common Mistakes:

• Thinking design increases disk space unnecessarily
• Believing all data must be in separate collections
• Assuming design stops data updates

2. Which of the following is the correct way to embed an address inside a user document in MongoDB?

easy

A. { name: 'Alice', address: ['NY', '10001'] }

B. { name: 'Alice', address: { city: 'NY', zip: '10001' } }

C. { name: 'Alice', address: 'NY, 10001' }

D. { name: 'Alice', address: null }

Solution

1. Step 1: Recognize embedded document syntax

   Embedding means putting a document inside another document as a nested object.

2. Step 2: Identify correct nested object format

   { name: 'Alice', address: { city: 'NY', zip: '10001' } } uses a nested object with keys city and zip, which is correct for embedding.

3. Final Answer:

   { name: 'Alice', address: { city: 'NY', zip: '10001' } } -> Option B

4. Quick Check:

   Embedded document = nested object = { name: 'Alice', address: { city: 'NY', zip: '10001' } } [OK]

Hint: Embed data as nested objects, not arrays or strings [OK]

Common Mistakes:

• Using arrays instead of objects for embedded data
• Storing address as a plain string
• Leaving embedded fields null without reason

3. Given this user document:
{ _id: 1, name: 'Bob', orders: [{ id: 101, total: 50 }, { id: 102, total: 30 }] }
What will be the result of the query db.users.findOne({ _id: 1 })?

medium

A. null

B. { _id: 1, name: 'Bob' }

C. { _id: 1, name: 'Bob', orders: [{ id: 101, total: 50 }, { id: 102, total: 30 }] }

D. SyntaxError

Solution

1. Step 1: Understand findOne query behavior

   The findOne query returns the entire document matching the filter {_id: 1}.

2. Step 2: Check document structure

   The document includes the orders array embedded inside, so the full document is returned.

3. Final Answer:

   { _id: 1, name: 'Bob', orders: [{ id: 101, total: 50 }, { id: 102, total: 30 }] } -> Option C

4. Quick Check:

   findOne returns full document = { _id: 1, name: 'Bob', orders: [{ id: 101, total: 50 }, { id: 102, total: 30 }] } [OK]

Hint: findOne returns full matching document [OK]

Common Mistakes:

• Expecting only part of the document returned
• Thinking query returns null if embedded arrays exist
• Confusing syntax errors with valid queries

4. You want to embed a list of comments inside a blog post document, but your code throws an error. Which is the likely cause?
{ title: 'Post', comments: 'Great post!' }

medium

A. Comments should be an array of objects, not a string.

B. Title field cannot be a string.

C. MongoDB does not allow embedding arrays.

D. The document must have an _id field.

Solution

1. Step 1: Check the comments field type

   Comments are given as a string, but embedding multiple comments requires an array of objects.

2. Step 2: Understand embedding requirements

   Embedding multiple related items means using an array of objects, not a single string.

3. Final Answer:

   Comments should be an array of objects, not a string. -> Option A

4. Quick Check:

   Embed lists as arrays, not strings [OK]

Hint: Embed lists as arrays of objects, not strings [OK]

Common Mistakes:

• Using string instead of array for multiple items
• Thinking title cannot be string
• Believing MongoDB forbids arrays
• Assuming _id is always required manually

5. You have a product catalog where each product has many reviews. Reviews can grow large over time. What is the best document design to handle this efficiently?

hard

A. Embed all reviews inside each product document.

B. Duplicate product data inside each review document.

C. Store only the first review inside the product document.

D. Store reviews in a separate collection linked by product ID.

Solution

1. Step 1: Consider document size limits and growth

   Embedding many reviews inside a product can make the document very large and slow to update.

2. Step 2: Choose design for large growing data

   Storing reviews separately and linking by product ID keeps product documents small and queries efficient.

3. Final Answer:

   Store reviews in a separate collection linked by product ID. -> Option D

4. Quick Check:

   Large growing data = separate collection [OK]

Hint: Large growing lists? Use separate collections [OK]

Common Mistakes:

• Embedding large growing arrays inside documents
• Storing only partial data inside main document
• Duplicating product data unnecessarily