Arrays in documents in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with array containing a specific value

Given a collection products where each document has an array field tags, what is the result of this query?

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

Assume the collection contains:

{ _id: 1, tags: ["organic", "fresh"] }
{ _id: 2, tags: ["imported", "fresh"] }
{ _id: 3, tags: ["organic", "local"] }

MongoDB

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

A [{ _id: 1, tags: ["organic", "fresh"] }, { _id: 3, tags: ["organic", "local"] }]

B [{ _id: 2, tags: ["imported", "fresh"] }]

C []

D [{ _id: 1, tags: ["organic", "fresh"] }, { _id: 2, tags: ["imported", "fresh"] }, { _id: 3, tags: ["organic", "local"] }]

💡 Hint

The query matches documents where the array contains the exact value.

The query matches documents where the 'tags' array contains the string 'organic'. Documents 1 and 3 have 'organic' in their tags, so they are returned.

❓ query_result

intermediate

Query documents with array length condition

What documents will this query return?

db.orders.find({ items: { $size: 3 } })

Assuming the collection orders has:

{ _id: 1, items: ["apple", "banana", "orange"] }
{ _id: 2, items: ["apple"] }
{ _id: 3, items: ["banana", "orange", "grape", "pear"] }

MongoDB

db.orders.find({ items: { $size: 3 } })

A [{ _id: 2, items: ["apple"] }]

B [{ _id: 1, items: ["apple", "banana", "orange"] }]

C [{ _id: 3, items: ["banana", "orange", "grape", "pear"] }]

D [{ _id: 1, items: ["apple", "banana", "orange"] }, { _id: 3, items: ["banana", "orange", "grape", "pear"] }]

💡 Hint

The $size operator matches arrays with exactly the specified length.

Only document 1 has an 'items' array with exactly 3 elements, so only it is returned.

📝 Syntax

advanced

Identify the syntax error in array update query

Which option contains a syntax error when trying to add a new element to the tags array in a document?

Query intent: Add "sale" to the tags array for document with _id: 5.

MongoDB

db.products.updateOne({ _id: 5 }, { $push: { tags: "sale" } })

A db.products.updateOne({ _id: 5 }, { $push: { tags: { $add: "sale" } } })

B db.products.updateOne({ _id: 5 }, { $push: { tags: ["sale"] } })

C db.products.updateOne({ _id: 5 }, { $push: { tags: { $each: ["sale"] } } })

D db.products.updateOne({ _id: 5 }, { $push: { tags: "sale" } })

💡 Hint

Check the operator used inside $push.

Option A uses an invalid operator '$add' inside $push, which causes a syntax error. The correct operator to add elements is $each or directly pushing a value.

❓ query_result

advanced

Find documents where array contains all specified elements

What documents will this query return?

db.recipes.find({ ingredients: { $all: ["flour", "sugar"] } })

Given the collection recipes:

{ _id: 1, ingredients: ["flour", "sugar", "eggs"] }
{ _id: 2, ingredients: ["flour", "butter"] }
{ _id: 3, ingredients: ["sugar", "flour"] }

MongoDB

db.recipes.find({ ingredients: { $all: ["flour", "sugar"] } })

A []

B [{ _id: 2, ingredients: ["flour", "butter"] }]

C [{ _id: 1, ingredients: ["flour", "sugar", "eggs"] }]

D [{ _id: 1, ingredients: ["flour", "sugar", "eggs"] }, { _id: 3, ingredients: ["sugar", "flour"] }]

💡 Hint

The $all operator matches arrays containing all specified elements, regardless of order.

Documents 1 and 3 both have 'flour' and 'sugar' in their ingredients arrays, so they match the query.

🧠 Conceptual

expert

Understanding atomicity of array updates in MongoDB

Which statement about updating arrays in MongoDB is true?

A MongoDB does not support atomic updates on arrays; each element update is separate and can partially fail.

B Array updates are atomic only if the array has less than 10 elements.

C Updating multiple elements in an array within a single document is atomic; either all changes succeed or none do.

D Atomicity applies only when updating arrays in multiple documents, not within a single document.

💡 Hint

Think about MongoDB's document-level atomicity.

MongoDB guarantees atomicity at the single document level, so all updates to arrays within one document happen atomically.