A module for defining custom functions that can be called in Ash expressions.

For example:

defmodule MyApp.Expressions.LevenshteinDistance do
  use Ash.CustomExpression,
    name: :levenshtein,
    arguments: [
      [:string, :string]
    ]

  def expression(AshPostgres.DataLayer, [left, right]) do
    {:ok, expr(fragment("levenshtein(?, ?)", left, right))}
  end

  # It is good practice to always define an expression for `Ash.DataLayer.Simple`,
  # as that is what Ash will use to run your custom expression in Elixir.
  # This allows us to completely avoid communicating with the database in some cases.

  def expression(data_layer, [left, right]) when data_layer in [
    Ash.DataLayer.Ets,
    Ash.DataLayer.Simple
  ] do
    {:ok, expr(fragment(&__MODULE__.levenshtein/2, left, right))}
  end

  # always define this fallback clause as well
  def expression(_data_layer, _args), do: :unknown

  @doc "Computes the levenshtein distance of two strings"
  def levenshtein(left, right) do
    # ......
  end
end

Options

• name - The name of the custom expression. This is the name that will be used in Ash expressions.
• arguments - A list of lists of types that the custom expression accepts. Each list represents a set of arguments that the custom expression can accept.
• predicate? - Whether this expression can be exposed as a predicate in filter interfaces. Defaults to false.

Registering Your Expression

Use compile-time configuration to register your custom expressions

config :ash, :custom_expressions, [MyApp.Expressions.LevenshteinDistance]