# 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.
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.
### Examples
```elixir
validate present(:other_number), where: absent(:that_number)
```
```elixir
validate present(:other_number) do
where {MyApp.Validations.IsPrime, attribute: :foo}
end
```
```elixir
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:
```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
```
