View Source Validations

Validations are similar to changes, except they cannot modify the changeset. They can only continue, or add an error.

Builtin Validations

There are a number of builtin validations that can be used, and are automatically imported into your resources. See Ash.Resource.Validation.Builtins for more.

Some examples of usage of builtin validations

validate match(:email, ~r/@/)

validate compare(:age, greater_than_or_equal_to: 18) do
  message "must be over 18 to sign up"

validate present(:last_name) do
  where [present(:first_name), present(:middle_name)]
  message "must also be supplied if setting first name and middle_name"

Custom Validations

defmodule MyApp.Validations.IsPrime do
  # transform and validate opts

  use Ash.Resource.Validation

  @impl true
  def init(opts) do
    if is_atom(opts[:attribute]) do
      {:ok, opts}
      {:error, "attribute must be an atom!"}

  @impl true
  def validate(changeset, opts, _context) do
    value = Ash.Changeset.get_attribute(changeset, opts[:attribute])
    # this is a function I made up for example
    if is_nil(value) || Math.is_prime?(value) do
      # The returned error will be passed into `Ash.Error.to_ash_error/3`
      {:error, field: opts[:attribute], message: "must be prime"}

This could then be used in a resource via:

validate {MyApp.Validations.IsPrime, attribute: :foo}

Anonymous Function Validations

You can also use anonymous functions for validations. Keep in mind, these cannot be made atomic. This is great for prototyping, but we generally recommend using a module, both for organizational purposes, and to allow adding atomic behavior.

validate fn changeset, _context ->
  # put your code here


The where can be used to perform validations conditionally.

The value of the where option can either be a validation or a list of validations. All of the where-validations must first pass for the main validation to be applied. For expressing complex conditionals, passing a list of built-in validations to where can serve as an alternative to writing a custom validation module.


validate present(:other_number), where: absent(:that_number)
validate present(:other_number) do
  where {MyApp.Validations.IsPrime, attribute: :foo}
validate present(:other_number),
  where: [
    numericality(:large_number, greater_than: 100),
    one_of(:magic_number, [7, 13, 123])

Action vs Global Validations

You can place a validation in any create, update, or destroy action. For example:

actions do
  create :create do
    validate compare(:age, greater_than_or_equal_to: 18)

Or you can use the global validations block to validate on all actions of a given type. Where statements can be used in either. Note the warning about running on destroy actions below.

validations do
  validate present([:foo, :bar], at_least: 1) do
    on [:create, :update]
    where present(:baz)

The validations section allows you to add validations across multiple actions of a changeset

Running on destroy actions

By default, validations in the global validations block will run on create and update only. Many validations don't make sense in the context of destroys. To make them run on destroy, use on: [:create, :update, :destroy]


validations do
  validate present([:foo, :bar]), on: :update
  validate present([:foo, :bar, :baz], at_least: 2), on: :create
  validate present([:foo, :bar, :baz], at_least: 2), where: [action_is(:action1, :action2)]
  validate absent([:foo, :bar, :baz], exactly: 1), on: [:update, :destroy]
  validate {MyCustomValidation, [foo: :bar]}, on: :create

Atomic Validations

To make a validation atomic, you have to implement the Ash.Resource.Validation.atomic/3 callback. This callback returns an atomic instruction, or a list of atomic instructions, or an error/indication that the validation cannot be done atomically. For our IsPrime example above, this would look something like:

defmodule MyApp.Validations.IsPrime do
  # transform and validate opts

  use Ash.Resource.Validation


  def atomic(changeset, opts, context) do
    # lets ignore that there is no easy/built-in way to check prime numbers in postgres
      # the list of attributes that are involved in the validation
      # the condition that should cause the error
      # here we refer to the new value or the current value
      expr(not(fragment("is_prime(?)", ^atomic_ref(opts[:attribute])))),
      # the error expression
        error(^InvalidAttribute, %{
          field: ^opts[:attribute],
          # the value that caused the error
          value: ^atomic_ref(opts[:attribute]),
          # the message to display
          message: ^(context.message || "%{field} must be prime"),
          vars: %{field: ^opts[:attribute]}

In some cases, validations operate on arguments only and therefore have no need of atomic behavior. for this, you can call validate/3 directly from atomic/3. The builtin Ash.Resource.Validation.Builtins.argument_equals/2 validation does this, for example.

@impl true
def atomic(changeset, opts, context) do
  validate(changeset, opts, context)