dry_v_either_filled.rb

require 'dry-validation'

schema = Dry::Validation.Schema do
  configure do
    def self.messages
      super.merge(en: { errors: { foo: 'either a or b can be filled' } })
    end
  end

  required(:a).maybe
  required(:b).maybe

  rule(foo: %i[a b]) do |a, b|
    a.filled? ^ b.filled?
  end
end

puts schema.(a: 'huh', b: 'heh').errors.inspect
# {:foo=>["either a or b can be filled"]}
puts schema.(a: 'huh', b: nil).errors.inspect
# {}
puts schema.(a: nil, b: 'heh').errors.inspect
# {}