Support custom error messages in custom validators (#621)

* Add custom message for custom validators

lib/open_api_spex/cast/error.ex

@@ -29,6 +29,7 @@ defmodule OpenApiSpex.Cast.Error do
   @type one_of_error :: {:one_of, [String.t()]}
   @type unexpected_field_error :: {:unexpected_field, String.t() | atom()}
   @type unique_items_error :: {:unique_items}
+  @type custom_error :: {:custom, String.t()}
 
   @type reason ::
           :all_of
@@ -56,6 +57,7 @@ defmodule OpenApiSpex.Cast.Error do
           | :one_of
           | :unexpected_field
           | :unique_items
+          | :custom
 
   @type args ::
           all_of_error()
@@ -84,6 +86,7 @@ defmodule OpenApiSpex.Cast.Error do
           | one_of_error()
           | unexpected_field_error()
           | unique_items_error()
+          | custom_error()
 
   @type t :: %__MODULE__{
           reason: reason(),
@@ -244,6 +247,11 @@ defmodule OpenApiSpex.Cast.Error do
     |> add_context_fields(ctx)
   end
 
+  def new(ctx, {:custom, message}) do
+    %__MODULE__{reason: :custom, meta: %{message: message}}
+    |> add_context_fields(ctx)
+  end
+
   @spec message(t()) :: String.t()
 
   def message(%{reason: :invalid_schema_type, type: type}) do
@@ -365,6 +373,10 @@ defmodule OpenApiSpex.Cast.Error do
     "Object property count #{meta.property_count} is less than minProperties: #{meta.min_properties}"
   end
 
+  def message(%{reason: :custom, meta: meta}) do
+    meta.message
+  end
+
   def message_with_path(error) do
     prepend_path(error, message(error))
   end

test/cast_test.exs

@@ -222,6 +222,34 @@ defmodule OpenApiSpec.CastTest do
       assert Error.message_with_path(error) == "#/age: Invalid strict_integer. Got: string"
     end
 
+    test "cast custom error with custom validator" do
+      defmodule EvenInt do
+        require OpenApiSpex
+
+        alias OpenApiSpex.Cast
+
+        OpenApiSpex.schema(%{
+          description: "An even integer",
+          type: :integer,
+          "x-validate": __MODULE__
+        })
+
+        def cast(context = %Cast{value: value}) when is_integer(value) and rem(value, 2) == 0,
+          do: Cast.ok(context)
+
+        def cast(context), do: Cast.error(context, {:custom, "Must be an even integer"})
+      end
+
+      schema = %Schema{type: :object, properties: %{even_number: EvenInt.schema()}}
+
+      assert {:error, errors} = cast(value: %{"even_number" => 1}, schema: schema)
+      assert [error] = errors
+      assert %Error{} = error
+      assert error.reason == :custom
+      assert error.path == [:even_number]
+      assert Error.message_with_path(error) == "#/even_number: Must be an even integer"
+    end
+
     test "nil value with xxxOf" do
       schema = %Schema{anyOf: [%Schema{nullable: true, type: :string}]}
       assert {:ok, nil} = cast(value: nil, schema: schema)