$slice modifier with $push in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $push operator do in MongoDB?

The $push operator adds a new value to an array field in a document. If the field doesn't exist, it creates the array with the new value.

beginner

What is the purpose of the $slice modifier when used with $push?

The $slice modifier limits the size of the array after pushing a new element. It keeps only a specified number of elements from the start or end of the array.

intermediate

How do you keep only the last 3 elements of an array when pushing a new element using $push and $slice?

Use $push with $each to add the element, and $slice: -3 to keep the last 3 elements. Example: { $push: { arr: { $each: [newValue], $slice: -3 } } }

intermediate

What happens if you use $slice: 0 with $push?

Using $slice: 0 removes all elements from the array after pushing, resulting in an empty array.

beginner

Can $slice be used alone without $push in an update operation?

No, $slice is a modifier used only with $push to control the array size after pushing elements. It cannot be used alone.

What does $push do in MongoDB?

A Removes a value from an array

B Adds a value to an array field

C Replaces the entire document

D Creates a new collection

$push adds a new element to an array field in a document.

How does $slice: -3 affect an array when used with $push?

A Removes all elements

B Keeps the first 3 elements

C Keeps the last 3 elements

D Adds 3 new elements

$slice: -3 keeps only the last 3 elements of the array after pushing.

Which of these is the correct syntax to push a value and limit array size to 5?

A { $slice: 5, $push: { arr: value } }

B { $push: { arr: value, $slice: 5 } }

C { $push: { arr: [value], $limit: 5 } }

D { $push: { arr: { $each: [value], $slice: 5 } } }

Use $each with $push and $slice to limit array size.

What happens if you use $slice: 0 with $push?

A Array becomes empty

B Array keeps all elements

C Array size doubles

D Operation fails

$slice: 0 removes all elements, resulting in an empty array.

Can $slice be used without $push?

A No, it only works with <code>$push</code>

B Yes, but only in aggregation

C Yes, but only with <code>$pull</code>

D Yes, it works alone

$slice is a modifier only for $push operations.

Explain how to use $push with $slice to keep an array's size limited after adding new elements.

Think about adding elements and then trimming the array.

Describe what happens when you use $slice: 0 with $push in a MongoDB update.

Consider what zero means for array length.