Array expressions ($size, $arrayElemAt, $filter) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $size array expression do in MongoDB?

The $size expression returns the number of elements in an array.

It helps you count how many items are inside an array field.

beginner

How does $arrayElemAt work in MongoDB?

$arrayElemAt returns the element at a specific position in an array.

Positions start at 0, so $arrayElemAt: [array, 0] gives the first item.

intermediate

Explain the purpose of $filter in MongoDB array expressions.

$filter lets you select only the elements in an array that match a condition.

It creates a new array with just the items you want.

intermediate

What happens if you use $size on a field that is not an array?

If the field is not an array (including null or missing), $size throws an error.

advanced

How can $filter be combined with $size to count filtered elements?

You can use $filter to create a smaller array with only matching elements, then use $size to count how many matched.

What does $size return when used on an array?

A The last element of the array

B The number of elements in the array

C A filtered version of the array

D The first element of the array

$size counts how many elements are inside an array.

Which expression would you use to get the third element of an array?

A <code>$filter</code>

B <code>$size</code>

C <code>$arrayElemAt</code> with index 2

D <code>$arrayElemAt</code> with index 3

Arrays start at index 0, so the third element is at index 2.

What does $filter do in MongoDB?

A Selects elements matching a condition

B Returns an element at a position

C Counts elements in an array

D Sorts the array elements

$filter creates a new array with only elements that meet a condition.

What will happen if $size is used on a non-array field?

A Returns 0

B Returns null

C Returns 1

D Throws an error

$size expects an array and throws an error if given something else.

How can you count only the elements in an array that meet a condition?

A Use <code>$filter</code> then <code>$size</code>

B Use <code>$arrayElemAt</code> with <code>$size</code>

C Use <code>$size</code> alone

D Use <code>$filter</code> alone

First filter the array, then count the filtered elements with $size.

Describe how to use $filter to select elements from an array based on a condition.

Think about how you pick only certain items from a list.

Explain how $arrayElemAt and $size can be used together in a query.

One picks an item, the other counts items.