Querying array elements directly in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with a specific array element

Given a collection products where each document has an array field tags, which query returns all documents where tags contains the string "organic"?

MongoDB

db.products.find({tags: "organic"})

A db.products.find({tags: {$elemMatch: { $eq: "organic" }}})

B db.products.find({tags: "organic"})

C db.products.find({tags: {$in: ["organic"]}})

D db.products.find({tags: {$all: ["organic"]}})

💡 Hint

Check how MongoDB matches array elements directly by value.

In MongoDB, querying an array field with a value directly matches documents where the array contains that value. So {tags: "organic"} finds documents where tags includes "organic".

Other operators like $elemMatch, $in, and $all have different purposes and are not needed here.

❓ query_result

intermediate

Query documents with array elements matching a condition

Given a collection orders where each document has an array items with subdocuments containing price, which query returns documents where at least one item has a price greater than 100?

MongoDB

db.orders.find({"items.price": {$gt: 100}})

A db.orders.find({items: {$elemMatch: {price: {$gt: 100}}}})

B db.orders.find({items: {$all: [{price: {$gt: 100}}]}})

C db.orders.find({items: {price: {$gt: 100}}})

D db.orders.find({"items.price": {$gt: 100}})

💡 Hint

Use dot notation to query array subdocuments directly.

Using "items.price": {$gt: 100} queries documents where any element in the items array has a price greater than 100.

Option D also works but is more verbose. Option D is invalid because items is an array, so matching a subdocument directly without $elemMatch or dot notation won't work. Option D is incorrect because $all expects exact matches.

📝 Syntax

advanced

Identify the syntax error in querying array elements

Which query will cause a syntax error when trying to find documents where the array scores contains a value less than 50?

A db.students.find({scores: {$lt 50}})

B db.students.find({scores: {$elemMatch: {$lt: 50}}})

C db.students.find({scores: {$lt: 50}})

D db.students.find({"scores": {$lt: 50}})

💡 Hint

Check the syntax of the comparison operator inside the query object.

Option A is missing a colon between $lt and 50, causing a syntax error.

Options A, B, and C are valid MongoDB queries for this purpose.

❓ optimization

advanced

Optimize query for array element existence

You want to find documents in users where the array roles contains the value "admin". Which query is the most efficient?

A db.users.find({roles: "admin"})

B db.users.find({roles: {$elemMatch: {$eq: "admin"}}})

C db.users.find({roles: {$all: ["admin"]}})

D db.users.find({roles: {$in: ["admin"]}})

💡 Hint

Direct matching on array elements is usually fastest.

Querying with {roles: "admin"} directly matches documents where the array contains "admin" and is the simplest and most efficient.

Using $elemMatch or $in adds unnecessary complexity and overhead.

🧠 Conceptual

expert

Understanding array element matching behavior

Consider a collection events where each document has an array participants with subdocuments containing name and age. Which query returns documents where there is a participant named "Alice" who is exactly 30 years old?

A db.events.find({"participants.name": "Alice", "participants.age": 30})

B db.events.find({participants: {name: "Alice", age: 30}})

C db.events.find({participants: {$elemMatch: {name: "Alice", age: 30}}})

D db.events.find({participants: {$all: [{name: "Alice"}, {age: 30}]}})

💡 Hint

Use $elemMatch to match multiple conditions on the same array element.

Option C uses $elemMatch to ensure both conditions apply to the same participant object.

Option C tries to match the entire array element exactly, which is unlikely to match unless the object is exactly {name: "Alice", age: 30} with no extra fields.

Option C matches documents where any participant has name "Alice" and any participant (not necessarily the same) has age 30, which is not guaranteed to be the same participant.

Option C requires the array to contain both objects separately, which is not the intended meaning.

Practice

1. What does the MongoDB query { tags: "mongodb" } do when applied to a collection where tags is an array field?

easy

A. Finds documents where the tags array contains the value "mongodb".

B. Finds documents where the tags array is exactly equal to "mongodb".

C. Finds documents where the tags array is empty.

D. Finds documents where the tags field does not exist.

Solution

1. Step 1: Understand array field querying in MongoDB

   When querying an array field with a value, MongoDB checks if the array contains that value anywhere.

2. Step 2: Analyze the query { tags: "mongodb" }

   This query looks for documents where the tags array includes the string "mongodb" as one of its elements.

3. Final Answer:

   Finds documents where the tags array contains the value "mongodb". -> Option A

4. Quick Check:

   Querying array with value checks for presence = B [OK]

Hint: Querying array with value checks for presence [OK]

Common Mistakes:

• Thinking it matches exact array equality
• Assuming it matches empty arrays
• Confusing missing field with array content

2. Which of the following is the correct MongoDB query syntax to find documents where the array field scores contains the number 85?

easy

A. { scores: { $contains: 85 } }

B. { scores: { $in: 85 } }

C. { scores: 85 }

D. { scores: { $eq: [85] } }

Solution

1. Step 1: Recall MongoDB syntax for matching array elements

   To find documents where an array contains a value, simply use { field: value } syntax.

2. Step 2: Evaluate each option

   { scores: { $contains: 85 } } uses a non-existent operator $contains. { scores: { $eq: [85] } } incorrectly uses $eq with an array. { scores: { $in: 85 } } uses $in incorrectly without an array. { scores: 85 } correctly uses { scores: 85 }.

3. Final Answer:

   { scores: 85 } -> Option C

4. Quick Check:

   Simple value match syntax = D [OK]

Hint: Use {field: value} to match array elements directly [OK]

Common Mistakes:

• Using non-existent operators like $contains
• Misusing $eq with arrays
• Passing non-array to $in operator

3. Given the collection documents:

{ _id: 1, scores: [70, 85, 90] }

{ _id: 2, scores: [60, 75] }

{ _id: 3, scores: [85, 95] }

What will be the result of the query { scores: 85 }?

medium

A. [] (empty array)

B. [{ _id: 2, scores: [60, 75] }]

C. Syntax error

D. [{ _id: 1, scores: [70, 85, 90] }, { _id: 3, scores: [85, 95] }]

Solution

1. Step 1: Identify which documents have 85 in their scores array

   Document 1 has scores [70, 85, 90] which includes 85. Document 3 has scores [85, 95] which also includes 85. Document 2 does not have 85.

2. Step 2: Understand the query result

   The query { scores: 85 } returns all documents where the scores array contains 85, so documents 1 and 3.

3. Final Answer:

   [{ _id: 1, scores: [70, 85, 90] }, { _id: 3, scores: [85, 95] }] -> Option D

4. Quick Check:

   Documents with 85 in scores = C [OK]

Hint: Query returns docs where array contains value [OK]

Common Mistakes:

• Expecting only one document
• Thinking query returns empty if multiple matches
• Confusing syntax error with valid query

4. Consider this incorrect MongoDB query to find documents where the tags array contains both "red" and "blue":

{ tags: { $all: "red", "blue" } }

What is the main issue with this query?

medium

A. The $all operator requires an array of values, not separate arguments.

B. The query should use $elemMatch instead of $all.

C. The field name should be inside quotes.

D. The query is missing a $and operator.

Solution

1. Step 1: Understand $all operator syntax

   The $all operator expects an array of values to match all elements inside the array field.

2. Step 2: Identify the syntax error in the query

   The query incorrectly passes separate arguments to $all instead of an array. Correct syntax is { tags: { $all: ["red", "blue"] } }.

3. Final Answer:

   The $all operator requires an array of values, not separate arguments. -> Option A

4. Quick Check:

   $all needs array syntax = A [OK]

Hint: Use array syntax with $all operator [OK]

Common Mistakes:

• Passing multiple values without array brackets
• Confusing $all with $elemMatch
• Ignoring syntax errors in operator usage

5. You want to find documents where the ratings array contains at least one element greater than 4 and less than 7. Which query correctly uses $elemMatch to achieve this?

hard

A. { ratings: { $gt: 4, $lt: 7 } }

B. { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } }

C. { ratings: { $in: [5, 6] } }

D. { ratings: { $elemMatch: { $gte: 4, $lte: 7 } } }

Solution

1. Step 1: Understand $elemMatch usage for multiple conditions on array elements

   $elemMatch allows specifying multiple conditions that must be true for the same array element.

2. Step 2: Analyze each option for correctness

   { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } } correctly uses $elemMatch with $gt and $lt to find elements >4 and <7. { ratings: { $gt: 4, $lt: 7 } } is invalid syntax because $gt and $lt cannot be used directly on the array field. { ratings: { $in: [5, 6] } } matches specific values but does not cover the range condition. { ratings: { $elemMatch: { $gte: 4, $lte: 7 } } } uses $gte and $lte which includes 4 and 7, not strictly greater and less.

3. Final Answer:

   { ratings: { $elemMatch: { $gt: 4, $lt: 7 } } } -> Option B

4. Quick Check:

   Use $elemMatch for multiple conditions on one element = A [OK]

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

Common Mistakes:

• Using $gt and $lt directly on array field
• Using $in instead of range operators
• Confusing inclusive and exclusive range operators