In MongoDB, the $slice aggregation pipeline operator returns a subset of an array.
To use $slice, you specify the number of elements to return from the array. You can also specify a starting position for which to take the subset from the array.
aggregation
MongoDB $size
In MongoDB, the $size aggregation pipeline operator counts and returns the total number of items in an array.
The $size operator accepts one argument. The argument can be any valid expression that resolves to an array.
MongoDB $setUnion
In MongoDB, the $setUnion aggregation pipeline operator accepts two or more arrays and returns an array containing the elements that appear in any of those input arrays.
$setUnion accepts two or more arguments, all of which can be any valid expression as long as they each resolve to an array. $setUnion treats the arrays as sets.
MongoDB $setIsSubset
In MongoDB, the $setIsSubset aggregation pipeline operator accepts two arrays and returns true when the first array is a subset of the second, and false when it isn’t.
The first array is also considered a subset when it equals the second array.
MongoDB $setIntersection
In MongoDB, the $setIntersection aggregation pipeline operator accepts two or more arrays and returns an array that contains the elements that appear in all input arrays.
$setIntersection accepts two or more arguments, all of which can be any valid expression as long as they each resolve to an array. $setIntersection treats the arrays as sets.
MongoDB $setEquals
In MongoDB, the $setEquals aggregation pipeline operator compares two or more arrays and returns true if they have the same distinct elements and false otherwise.
$setEquals accepts two or more arguments, all of which can be any valid expression as long as they each resolve to an array. $setEquals treats the arrays as sets.
MongoDB $setDifference
In MongoDB, the $setDifference aggregation pipeline operator accepts two sets and performs a relative complement of the second set relative to the first. It returns an array containing the elements that only exist in the first set.
$setDifference accepts two arguments, both of which can be any valid expression as long as they each resolve to an array. $setDifference treats the arrays as sets.
7 Ways to Count Documents in MongoDB
MongoDB offers various methods for counting the documents in a collection or view. There are also some aggregation operators that enable you to count incoming documents from a previous aggregation pipeline stage.
MongoDB $sampleRate
In MongoDB, the $sampleRate aggregation pipeline stage matches a random selection of input documents.
The number of documents selected approximates the sample rate expressed as a percentage of the total number of documents.
The $sampleRate operator was introduced in MongoDB 4.4.2.
MongoDB $reverseArray
In MongoDB, the $reverseArray aggregation pipeline operator reverses the order of items in an array.
It accepts an array expression as an argument and returns an array with the elements in reverse order.