Arrays in documents in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the 'tags' array contains the value 'mongodb'.

MongoDB

db.collection.find({ tags: [1] })

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

A"mongodb"

B{ "$in": ["mongodb"] }

C{ "$all": ["mongodb"] }

D{ "$elemMatch": { "$eq": "mongodb" } }

Attempts:

3 left

💡 Hint

Common Mistakes

Using $in or $all unnecessarily when a direct value works.

Using incorrect query operators that don't match the array content.

To find documents where the array contains a specific value, you can directly query with that value. Here, 'tags: "mongodb"' matches documents where 'tags' array includes 'mongodb'.

2fill in blank

medium

Complete the code to find documents where the 'scores' array contains at least one value greater than 80.

MongoDB

db.collection.find({ scores: { [1]: 80 } })

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' instead of 'greater than'.

Using $eq which checks for equality, not greater than.

The operator $gt means 'greater than'. So, this query finds documents where the 'scores' array has at least one element greater than 80.

3fill in blank

hard

Fix the error in the query to find documents where the 'comments' array contains an object with 'author' equal to 'Alice'.

MongoDB

db.collection.find({ comments: { [1]: { author: "Alice" } } })

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

A$size

B$in

C$all

D$elemMatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using $in which matches values but not objects by fields.

Using $all which requires all conditions to be met by array elements.

The $elemMatch operator matches documents where at least one element in the array matches the specified query. Here, it finds documents with a 'comments' array containing an object with 'author' equal to 'Alice'.

4fill in blank

hard

Fill both blanks to find documents where the 'ratings' array has exactly 3 elements and contains the value 5.

MongoDB

db.collection.find({ ratings: { [1]: 3, [2]: [5] } })

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

A$size

B$eq

C$in

D$gt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $eq instead of $in to check for value presence in array.

Using $gt which checks for greater than, not array length.

$size checks the array length, so $size: 3 means the array has exactly 3 elements. $in checks if the array contains the value 5.

5fill in blank

hard

Fill all three blanks to create a query that finds documents where the 'reviews' array contains an object with 'score' greater than 7 and 'verified' equal to true.

MongoDB

db.collection.find({ reviews: { [1]: { score: { [2]: 7 }, verified: { [3]: true } } } })

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

A$elemMatch

B$gt

Cverified

D$eq

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'verified' as a key inside $elemMatch without $eq operator.

Using $in or $all instead of $elemMatch for object matching.

$elemMatch matches array elements by multiple conditions. Inside it, $gt checks if 'score' is greater than 7, and $eq checks if 'verified' is true.

Practice

1. What is the main purpose of using arrays in MongoDB documents?

easy

A. To index documents automatically

B. To create multiple documents at once

C. To enforce data types on fields

D. To store multiple values in a single field

Solution

1. Step 1: Understand what arrays do in MongoDB

   Arrays allow storing multiple values inside one document field, like a list.

2. Step 2: Compare options with array purpose

   Only To store multiple values in a single field correctly describes storing multiple values in one field.

3. Final Answer:

   To store multiple values in a single field -> Option D

4. Quick Check:

   Arrays = multiple values in one field [OK]

Hint: Arrays hold many values in one field [OK]

Common Mistakes:

• Thinking arrays create multiple documents
• Confusing arrays with indexing
• Believing arrays enforce data types

2. Which of the following is the correct way to define an array field named tags in a MongoDB document?

easy

A. { tags: "mongodb, database" }

B. { tags: { "mongodb", "database" } }

C. { tags: ["mongodb", "database"] }

D. { tags: ("mongodb", "database") }

Solution

1. Step 1: Recall MongoDB array syntax

   Arrays in MongoDB are defined using square brackets [] with comma-separated values.

2. Step 2: Check each option's syntax

   { tags: ["mongodb", "database"] } uses square brackets correctly. Options A, B, and D use incorrect syntax for arrays.

3. Final Answer:

   { tags: ["mongodb", "database"] } -> Option C

4. Quick Check:

   Arrays use [] brackets [OK]

Hint: Arrays use square brackets [] in MongoDB [OK]

Common Mistakes:

• Using quotes instead of brackets for arrays
• Using curly braces {} which define objects
• Using parentheses () which are invalid for arrays

3. Given the document { name: "Alice", scores: [85, 90, 78] }, what will the query db.collection.find({ scores: 90 }) return?

medium

A. Documents where the scores array contains 90

B. Documents where scores equals exactly 90

C. Documents where scores is greater than 90

D. No documents because 90 is inside an array

Solution

1. Step 1: Understand MongoDB array matching

   Querying with { scores: 90 } matches documents where the array contains the value 90.

2. Step 2: Analyze the given document and query

   The scores array includes 90, so the document matches and will be returned.

3. Final Answer:

   Documents where the scores array contains 90 -> Option A

4. Quick Check:

   Query matches array elements directly [OK]

Hint: Query value matches any array element [OK]

Common Mistakes:

• Thinking query matches whole array only
• Assuming query checks for greater than
• Believing arrays block direct value matching

4. What is wrong with this MongoDB update query to add a new tag to the tags array?

db.collection.updateOne({ _id: 1 }, { $push: { tags: "new" } })

medium

A. The query is missing the filter document

B. The $push operator is used correctly; no error

C. The $push operator requires the value to be an array

D. The update document should use $addToSet instead of $push

Solution

1. Step 1: Understand $push operator usage

   $push adds a single value to an array field; it accepts a single value, not necessarily an array.

2. Step 2: Check the query structure

   The filter {_id: 1} is present, and $push is used correctly to add "new" to tags array.

3. Final Answer:

   The $push operator is used correctly; no error -> Option B

4. Quick Check:

   $push adds single values to arrays [OK]

Hint: $push accepts single values, no array needed [OK]

Common Mistakes:

• Thinking $push needs an array value
• Confusing $push with $addToSet for uniqueness
• Missing the filter document in update

5. You have documents with a field comments which is an array of objects like { user: "Bob", text: "Nice!" }. How do you write a query to find documents where comments contains an object with user equal to "Bob" and text containing the word "Nice"?

hard

A. { comments: { $elemMatch: { user: "Bob", text: /Nice/ } } }

B. { "comments.user": "Bob", "comments.text": /Nice/ }

C. { comments: { $all: [ { user: "Bob" }, { text: /Nice/ } ] } }

D. { comments: { $in: [ { user: "Bob", text: /Nice/ } ] } }

Solution

1. Step 1: Understand matching objects inside arrays

   $elemMatch matches array elements that satisfy all conditions inside it.

2. Step 2: Analyze each option for correct syntax

   { comments: { $elemMatch: { user: "Bob", text: /Nice/ } } } uses $elemMatch with both conditions together, correctly matching one object with user "Bob" and text matching /Nice/.

3. Final Answer:

   { comments: { $elemMatch: { user: "Bob", text: /Nice/ } } } -> Option A

4. Quick Check:

   $elemMatch matches array elements with multiple conditions [OK]

Hint: Use $elemMatch for multiple conditions on one array element [OK]

Common Mistakes:

• Using separate field queries (dot notation) which match conditions across different elements
• Using $all which matches separate elements, not one
• Using $in which matches exact elements, not partial fields