Validate keyrelay relative and absolute in controller

app/controllers/epp_controller.rb

@@ -75,6 +75,23 @@ class EppController < ApplicationController
     epp_errors.empty?
   end
 
+  # let's follow grape's validations: https://github.com/intridea/grape/#parameter-validation-and-coercion
+  def exactly_one_of(*selectors)
+    present_count = 0
+    selectors.each do |selector|
+      full_selector = [@prefix, selector].join(' ')
+      el = params[:parsed_frame].css(full_selector).first
+      present_count += 1 if el && el.text.present?
+    end
+
+    return if present_count == 1
+
+    epp_errors << {
+      code: '2003',
+      msg: I18n.t(:exactly_one_parameter_required, params: selectors.join(' or '))
+    }
+  end
+
   def xml_attrs_present?(ph, attributes) # TODO: THIS IS DEPRECATED AND WILL BE REMOVED IN FUTURE
     attributes.each do |x|
       epp_errors << {