set.fold()

Reduce a Set to a single, accumulated value by applying a function to each element. Iterates through elements from left to right. Uses a provided seed as the initial value.

Signature

fold(seed: B, reducer: (B, A) => B) => B

Description

The fold() method 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.

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:

Parameters

Parameter Type Required Description

seed

Any

Yes

Initial accumulator value provided to the reducer function.

reducer

Function

Yes

Anonymous FQL function to call on each element of the Set.

reducer parameters:

Parameter Type Required Description

accumulator

Any

Yes

Value returned by the previous reducer function call. On the first call, seed is passed as the accumulator.

current

Any

Yes

The current element’s value.

Return value

Type Description

Any

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.
  let data: Any = set
  data.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.
  let data: Any = set
  data.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:

For additional examples using the groupBy() UDF, see Group By: Aggregate data in Fauna.

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