feat: attribute_in/2 builtin validation

lib/ash/resource/validation/attribute_in.ex

@@ -0,0 +1,47 @@
+defmodule Ash.Resource.Validation.AttributeIn do
+  @moduledoc false
+
+  @opt_schema [
+    attribute: [
+      type: :atom,
+      required: true,
+      doc: "The attribute to check"
+    ],
+    list: [
+      type: :any,
+      required: true,
+      doc: "The list of values that the attribute must be in"
+    ]
+  ]
+
+  use Ash.Resource.Validation
+  alias Ash.Error.Changes.InvalidAttribute
+
+  @impl true
+  def init(opts) do
+    case Spark.OptionsHelpers.validate(opts, @opt_schema) do
+      {:ok, opts} ->
+        {:ok, opts}
+
+      {:error, error} ->
+        {:error, Exception.message(error)}
+    end
+  end
+
+  @impl true
+  def validate(changeset, opts) do
+    value = Ash.Changeset.get_attribute(changeset, opts[:attribute])
+
+    if value in opts[:list] do
+      :ok
+    else
+      {:error,
+       InvalidAttribute.exception(
+         value: value,
+         field: opts[:attribute],
+         message: "must equal %{value}",
+         vars: [field: opts[:attribute], value: opts[:value]]
+       )}
+    end
+  end
+end

lib/ash/resource/validation/builtins.ex

@@ -88,6 +88,21 @@ defmodule Ash.Resource.Validation.Builtins do
     {Validation.AttributeEquals, attribute: attribute, value: value}
   end
 
+  @doc """
+  Validates that an attribute is being changed to one of a set of specific values, or is in the the given list if it is not being changed.
+
+  ## Examples
+
+      validate attribute_in(:state, [1, 2, 3])
+
+      # Or to only check for changing to a something in a given list
+      validate attribute_in(:state, [1, 2, 3]), where: [changing(:state)]
+  """
+  @spec attribute_in(attribute :: atom, list :: [term]) :: Validation.ref()
+  def attribute_in(attribute, list) do
+    {Validation.AttributeIn, attribute: attribute, list: list}
+  end
+
   @string_length_opts [
     min: [
       type: :non_neg_integer,