hanami_validations_bug.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  gem "rake"
  gem "hanami", github: "hanami"
  gem "i18n"
  gem "minitest"
end

require "minitest/autorun"
require "hanami/controller"
require "dry/validation/messages/i18n"

I18n.load_path = Dir["ru.yml"]
I18n.default_locale = "ru"
I18n.backend.load_translations

class ApplicationSchema < Dry::Validation::Schema
  configure do |config|
    config.messages = :i18n
  end
end

FooSchema = Dry::Validation.Schema(ApplicationSchema) do
  configure do
    def foo?(current)
      false
    end
  end

  required(:title) { foo? }
end

Hanami::Controller.configure do
  handle_exceptions false
end

class CreateFoo
  include Hanami::Action

  params do
    required(:foo).schema(FooSchema)
  end

  def call(params)
    params.errors
  end
end

class BugTest < Minitest::Test
  def  test_plain_schema
    params = { title: "OK" }
    action = CreateFoo.new
    assert_equal Hash[title: ["Что такое?"]], FooSchema.(params).errors
  end
  
  def test_controller_action
    params = Hash[foo: { title: "OK" }]
    action = CreateFoo.new
    response = action.call(params) # Dry::Validation::MissingMessageError: message for foo? was not found
    
    assert_equal response[0], 200
  end
end

ru.yml

ru:
  errors:
    foo?: Foo