Why querying nested data matters in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the nested field 'address.city' is 'New York'.

MongoDB

db.users.find({"address.[1]": "New York"})

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

Azip

Bstate

Ccity

Dstreet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'state' or 'zip' instead of 'city' will not find the correct documents.

Forgetting to use dot notation for nested fields.

The nested field 'address.city' is accessed using dot notation as 'address.city'. So the blank should be filled with 'city'.

2fill in blank

medium

Complete the code to find documents where the nested array 'orders.items' contains 'apple'.

MongoDB

db.orders.find({"orders.[1]": "apple"})

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

Aitems

Bproduct

Cquantity

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'product' instead of 'items' will not match the array field.

Not using dot notation for nested arrays.

To query an array field inside 'orders', we use 'orders.items'. So the blank should be 'items'.

3fill in blank

hard

Fix the error in the query to find documents where 'profile.age' is greater than 30.

MongoDB

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

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

A$lt

B$gt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using '$lt' will find ages less than 30, which is incorrect here.

Using '$eq' will find ages exactly 30, not greater.

The operator '$gt' means 'greater than'. To find ages greater than 30, use '$gt'.

4fill in blank

hard

Fill both blanks to find documents where 'comments.rating' is at least 4 and 'comments.approved' is true.

MongoDB

db.posts.find({"comments.rating": { [1]: 4 }, "comments.approved": [2] })

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

A$gte

Bfalse

Ctrue

D$lte

Attempts:

3 left

💡 Hint

Common Mistakes

Using '$lte' will find ratings less than or equal to 4, which is wrong here.

Setting approved to false will not find approved comments.

'$gte' means 'greater than or equal to', so it fits for rating at least 4. The approved field should be true, so use 'true'.

5fill in blank

hard

Fill all three blanks to create a projection that includes 'name', excludes '_id', and includes nested field 'profile.email'.

MongoDB

db.users.find({}, { [1]: 1, [2]: 0, [3]: 1 })

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

Aname

B_id

Cprofile.email

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Setting '_id' to 1 will include it, not exclude.

Forgetting to include 'profile.email' will omit the nested email field.

To include 'name' and 'profile.email', set them to 1. To exclude '_id', set it to 0.

Practice

1. What is the main reason to use dot notation when querying nested data in MongoDB?

easy

A. To access fields inside embedded documents

B. To update the entire document at once

C. To delete the whole collection

D. To create a new database

Solution

1. Step 1: Understand nested data structure

   Nested data means one document contains another document inside it.

2. Step 2: Use dot notation to access inner fields

   Dot notation lets you specify the path to a field inside the embedded document.

3. Final Answer:

   To access fields inside embedded documents -> Option A

4. Quick Check:

   Dot notation = access nested fields [OK]

Hint: Dot notation drills down into nested fields fast [OK]

Common Mistakes:

• Thinking dot notation updates whole documents
• Confusing dot notation with collection operations
• Using dot notation to create databases

2. Which of the following is the correct MongoDB query syntax to find documents where the nested field address.city equals "Paris"?

easy

A. { "address.city": "Paris" }

B. { address: { city: "Paris" } }

C. { address.city = "Paris" }

D. { address->city: "Paris" }

Solution

1. Step 1: Recall MongoDB query syntax for nested fields

   MongoDB uses dot notation inside quotes to query nested fields.

2. Step 2: Identify correct syntax

   { "address.city": "Paris" } uses "address.city" as a string key with value "Paris", which is correct.

3. Final Answer:

   { "address.city": "Paris" } -> Option A

4. Quick Check:

   Dot notation in quotes = correct query [OK]

Hint: Use quotes and dot notation for nested field queries [OK]

Common Mistakes:

• Using object syntax without quotes for nested fields
• Using '=' instead of ':' in query
• Using arrow '->' instead of dot notation

3. Given the collection documents:

{ "name": "Alice", "contact": { "email": "[e-mail]", "phone": "1234" } }, { "name": "Bob", "contact": { "email": "[e-mail]", "phone": "5678" } }

What will the query db.collection.find({ "contact.email": "[e-mail]" }) return?

medium

A. [{ "name": "Alice", "contact": { "email": "[e-mail]", "phone": "1234" } }]

B. [] (empty array)

C. [{ "name": "Bob", "contact": { "email": "[e-mail]", "phone": "5678" } }]

D. SyntaxError

Solution

1. Step 1: Understand the query condition

   The query looks for documents where the nested field contact.email equals "[e-mail]".

2. Step 2: Match documents in the collection

   Only Bob's document has contact.email as "[e-mail]", so it will be returned.

3. Final Answer:

   [{ "name": "Bob", "contact": { "email": "[e-mail]", "phone": "5678" } }] -> Option C

4. Quick Check:

   Query matches Bob's email = [{ "name": "Bob", "contact": { "email": "[e-mail]", "phone": "5678" } }] [OK]

Hint: Match exact nested field value with dot notation [OK]

Common Mistakes:

• Expecting all documents to return
• Confusing nested field with top-level field
• Thinking query causes syntax error

4. Consider this query: db.users.find({ contact.phone: "1234" }). What is the main error here?

medium

A. Query is correct and will run fine

B. Using wrong collection name

C. Phone number should be a number, not string

D. Missing quotes around nested field name

Solution

1. Step 1: Check syntax for nested field keys

   MongoDB requires nested field names with dots to be in quotes in queries.

2. Step 2: Identify missing quotes error

   The query uses contact.phone without quotes, causing syntax error.

3. Final Answer:

   Missing quotes around nested field name -> Option D

4. Quick Check:

   Nested keys need quotes = Missing quotes around nested field name [OK]

Hint: Always quote nested keys with dots in queries [OK]

Common Mistakes:

• Ignoring quotes around nested keys
• Assuming phone must be number type
• Thinking collection name causes error

5. You have documents with nested arrays like:

{ "name": "Eve", "orders": [ { "id": 1, "item": "book" }, { "id": 2, "item": "pen" } ] }

Which query finds documents where any order's item is "pen"?

hard

A. { "orders": { "item": "pen" } }

B. { "orders.item": "pen" }

C. { "orders[0].item": "pen" }

D. { "orders": [ { "item": "pen" } ] }

Solution

1. Step 1: Understand querying nested arrays

   MongoDB lets you query array elements using dot notation on the array field.

2. Step 2: Identify correct query for any matching array element

   { "orders.item": "pen" } uses "orders.item" which matches any element's item field equal to "pen".

3. Final Answer:

   { "orders.item": "pen" } -> Option B

4. Quick Check:

   Dot notation on array fields matches any element [OK]

Hint: Use dot notation on array fields to match any element [OK]

Common Mistakes:

• Trying to match whole array instead of elements
• Using array index in query which is invalid
• Using nested object without array context