$slice modifier with $push in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after using $push with $slice?

Consider a MongoDB document with an array field scores initially as [10, 20, 30]. We run this update:

{ $push: { scores: { $each: [40, 50], $slice: -3 } }}

What will be the scores array after this update?

A [30, 40, 50]

B [10, 20, 30, 40, 50]

C [20, 30, 40, 50]

D [40, 50]

💡 Hint

Remember that $slice with a negative number keeps the last N elements.

The $push with $each adds 40 and 50 to the array, making it [10, 20, 30, 40, 50]. Then $slice: -3 keeps only the last 3 elements, which are [30, 40, 50].

📝 Syntax

intermediate

Which update query is syntactically correct for $push with $slice?

Choose the syntactically valid MongoDB update query that uses $push with $slice to limit array size.

A { $push: { items: { $each: [1,2,3], $slice: -2 } } }

B { $push: { items: { $slice: -2, $each: [1,2,3] } } }

C { $push: { items: { $each: [1,2,3], $slice: 2 } } }

D { $push: { items: { $each: [1,2,3], $limit: 2 } } }

💡 Hint

The order of $each and $slice matters, and $limit is not a valid modifier.

Option A is correct because $each and $slice are used properly, and $slice can be negative to keep last N elements. Option A uses positive slice which keeps first N elements but is valid syntax too, but the question asks for correct syntax with negative slice. Option A has wrong order, and Option A uses invalid $limit.

❓ optimization

advanced

How to efficiently keep only the last 5 elements in an array using $push?

You want to add multiple elements to an array field logs and keep only the last 5 elements to save space. Which update query is the most efficient?

A { $push: { logs: { $each: newEntries, $sort: -1, $slice: 5 } } }

B { $push: { logs: { $each: newEntries } }, $slice: -5 }

C { $push: { logs: newEntries }, $slice: -5 }

D { $push: { logs: { $each: newEntries, $slice: -5 } } }

💡 Hint

Remember that $slice must be inside the $push modifier with $each.

Option D correctly uses $each to add multiple elements and $slice: -5 to keep the last 5 elements. Option D and C place $slice outside $push, which is invalid. Option D uses $sort which is not a valid modifier in this context.

🔧 Debug

advanced

Why does this $push update with $slice not work as expected?

Given this update query:

{ $push: { data: { $each: [5,6], $slice: 2 } }}

The array data ends up with only the first 2 elements instead of the last 2. Why?

A Because $slice must be negative to work with $each.

B Because $each must be after $slice in the query.

C Because $slice with positive number keeps the first N elements, not the last.

D Because $slice only works with $pop, not $push.

💡 Hint

Think about what positive and negative values for $slice mean.

A positive $slice keeps the first N elements of the array after pushing. To keep the last N elements, $slice must be negative. So the array keeps the first 2 elements, not the last 2.

🧠 Conceptual

expert

What happens if $slice is zero in a $push update?

In MongoDB, if you run this update:

{ $push: { arr: { $each: [7,8], $slice: 0 } }}

What will be the resulting arr array?

A The array will contain only [7,8].

B The array will be empty after the update.

C The array will remain unchanged.

D The update will cause a syntax error.

💡 Hint

Think about what $slice: 0 means for array length.

$slice: 0 means keep zero elements, so after pushing the new elements, the array is trimmed to zero length, resulting in an empty array.