$all operator for matching all elements in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where the 'tags' array contains both 'red' and 'blue'

Given a collection with documents containing a 'tags' array, which query returns documents where 'tags' contains both 'red' and 'blue'?

MongoDB

db.collection.find({ tags: { $all: ['red', 'blue'] } })

A db.collection.find({ tags: { $all: ['red', 'blue'] } })

B db.collection.find({ tags: { $in: ['red', 'blue'] } })

C db.collection.find({ tags: { $all: ['red'] } })

D db.collection.find({ tags: { $all: ['blue'] } })

💡 Hint

Use $all to match arrays containing all specified elements.

The $all operator matches arrays that contain all the specified elements. Option A correctly uses $all with both 'red' and 'blue'. Option A uses $in which matches any of the elements, not all. Options C and D only check for one element each.

🧠 Conceptual

intermediate

Understanding $all operator behavior with nested arrays

What does the $all operator do when matching documents where the field contains nested arrays? For example, if a document has field 'values': [[1,2], [3,4]], what does $all: [[1,2], [3,4]] match?

A Matches documents where 'values' contains any of the numbers 1, 2, 3, or 4

B Matches documents where 'values' contains both arrays [1,2] and [3,4] exactly

C Matches documents where 'values' contains arrays with any of the numbers 1, 2, 3, or 4 in any order

D Matches documents where 'values' contains a single array with all numbers 1, 2, 3, and 4

💡 Hint

Think about how $all matches elements exactly, including arrays.

The $all operator matches elements exactly. So if you specify $all: [[1,2], [3,4]], it looks for the array field to contain both arrays [1,2] and [3,4] as elements. It does not flatten or partially match numbers inside nested arrays.

📝 Syntax

advanced

Identify the syntax error in this $all query

Which option contains a syntax error in the MongoDB query using $all to match 'colors' containing 'green' and 'yellow'?

MongoDB

db.collection.find({ colors: { $all: 'green', 'yellow' } })

A db.collection.find({ colors: { $all: ['green', 'yellow'] } })

B db.collection.find({ colors: { $all: ['yellow'] } })

C db.collection.find({ colors: { $all: ['green'] } })

D db.collection.find({ colors: { $all: 'green', 'yellow' } })

💡 Hint

Check the syntax for $all operator's value type.

The $all operator requires an array as its value. Option D incorrectly provides multiple arguments instead of a single array, causing a syntax error. Options A, B, and C correctly use arrays.

❓ optimization

advanced

Optimizing a query using $all with multiple elements

You want to find documents where the 'features' array contains 'wifi', 'parking', and 'pool'. Which query is the most efficient and correct?

A db.collection.find({ $and: [ { features: 'wifi' }, { features: 'parking' }, { features: 'pool' } ] })

B db.collection.find({ features: { $in: ['wifi', 'parking', 'pool'] } })

C db.collection.find({ features: { $all: ['wifi', 'parking', 'pool'] } })

D db.collection.find({ features: 'wifi' }).find({ features: 'parking' }).find({ features: 'pool' })

💡 Hint

Use a single query operator that matches all elements at once.

Option C uses $all to match all specified elements in one query, which is efficient. Option C matches any of the elements, not all. Option C is invalid chaining of find(). Option C works but is less efficient than $all.

🔧 Debug

expert

Why does this $all query return no results?

You run this query: db.collection.find({ tags: { $all: ['apple', 'banana'] } }) but get no results, even though documents have 'tags' arrays with 'apple' and 'banana' but in nested arrays like ['apple', ['banana']] or [['apple'], 'banana']. Why?

A Because $all matches exact elements, nested arrays are not matched as individual elements

B Because $all only matches strings, not arrays

C Because the query syntax is incorrect and causes no matches

D Because $all requires the elements to be in order in the array

💡 Hint

Consider how $all matches elements exactly, including nested arrays.

The $all operator matches elements exactly. If the array contains nested arrays, the elements inside those nested arrays are not considered top-level elements. So 'banana' inside ['banana'] is not matched as 'banana' by $all.