# Validations

Validations are similar to [changes](/documentation/topics/resources/changes.md), 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

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

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

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

## Custom Validations

```elixir
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}
    else
      {:error, "attribute must be an atom!"}
    end
  end

  @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
      :ok
    else
      # The returned error will be passed into `Ash.Error.to_ash_error/3`
      {:error, field: opts[:attribute], message: "must be prime"}
    end
  end
end
```

This could then be used in a resource via:

```elixir
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.

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

## Where

The `where` can be used to perform validations conditionally. This functions by running the validation, and if the validation returns an error, we discard the error and skip the operation. This means that even custom validations can be used in conditions.

For example:

```elixir
validate present(:other_number) do
  where [{MyApp.Validations.IsPrime, attribute: :foo}]
end
```

## Action vs Global Validations

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

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

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.

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

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

> ### Running on destroy actions {: .warning}
>
> 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]`

### Examples

```elixir
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
end
```

## Atomic Validations

To make a validation atomic, you have to implement the `c: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:

```elixir
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
    {:atomic,
      # the list of attributes that are involved in the validation
      [opts[:attribute]],
      # 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
      expr(
        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]}
        })
      )
    )}
  end
end
```

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.

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