set.fold()
Description
Iterates through each element in a Set to perform a rolling operation. For
example, you can use fold()
to calculate a rolling sum, concatenate elements,
or perform complex transformations.
fold()
calls a reducer callback function on every element of the Set from
left to right. The reducer function takes two arguments:
-
The accumulator that holds the running result from previous iterations. For the first iteration, a seed value serves as the initial accumulator.
-
The current element’s value from the Set.
The method returns the result of the last iteration. The calling Set isn’t changed.
Eager loading
This method uses eager loading and requires a read of each document in the calling Set. For large Sets, this may result in poor performance and high costs.
Fold family methods
FQL supports several methods for folds, which iteratively reduce a Set to a single value. These methods include:
The methods are similar but have the following differences:
-
set.fold()
andset.foldRight()
accept an initial seed value and use it as the initial accumulator.set.reduce()
andset.reduceRight()
use the Set’s first element as the initial accumulator. -
set.fold()
andset.reduce()
iterate through the Set’s elements from left to right.set.foldRight()
andset.reduceRight()
iterate through the Set’s elements from right to left.
Parameters
Parameter | Type | Required | Description |
---|---|---|---|
seed |
Yes |
Initial accumulator value provided to the reducer function. |
|
reducer |
Yes |
Anonymous FQL function to call on each element of the Set. |
Return value
Type | Description |
---|---|
Result of the last reducer function call. For an empty Set, the seed is returned. |
Examples
Basic example
// `toSet()` converts an Array to a Set.
let set = [1, 2, 3].toSet()
set.fold(100, (value, elem) => value + elem)
106
Group by operation
FQL doesn’t provide a built-in GROUP BY
operation. However, you use fold()
in an
anonymous FQL function or a
user-defined function (UDF) to
achieve the same result.
As an FQL function:
// Defines an anonymous `groupBy()` function.
// `groupBy()` two arguments:
// * `set`: Set or Array containing data to group
// * `key_fn`: Grouping key for each element
let groupBy = (set, key_fn) => {
// Calls the `fold()` function on the `set`
// Set or Array.
set.fold(
// Start with an empty object.
{},
(acc, val) => {
// For each value, get a key using the `key_fn` arg.
let key = key_fn(val)
let existing_group = acc[key] ?? []
// Append the current value to the Set or
// Array for that key.
let new_group = existing_group.append(val)
let new_entry = Object.fromEntries([
[key, new_group]
])
// Return an object with grouped results.
Object.assign(acc, new_entry)
}
)
}
// Call the `groupBy()` function.
// Groups `Product` documents by category name.
groupBy(Product.all(), .category!.name)
You can also define a groupBy()
UDF. This lets you reuse
the function across multiple queries.
You create and manage a UDF as an FSL function schema:
// Defines the `groupBy()` UDF.
// `groupBy()` two arguments:
// * `set`: Set or Array containing data to group
// * `key_fn`: Grouping key for each element
function groupBy (set, key_fn) {
// Calls the `fold()` function on the `set`
// Set or Array.
set.fold(
// Start with an empty object.
{},
(acc, val) => {
// For each value, get a key using the `key_fn` arg.
let key: String = key_fn(val)
let existing_group = acc[key] ?? []
// Append the current value to the Set or
// Array for that key.
let new_group = existing_group.append(val)
let new_entry = Object.fromEntries([
[key, new_group]
])
// Return an object with grouped results.
Object.assign(acc, new_entry)
}
)
}
You can create and manage schema using any of the following:
-
The Fauna CLI
-
The Fauna Dashboard
-
The Fauna Core HTTP API’s Schema endpoints
For additional examples using the groupBy()
UDF, see Group By: Aggregate data in Fauna.
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