Add validation to ensure start date is before organisation's merge date if present

app/models/organisation_name_change.rb

@@ -10,6 +10,7 @@ class OrganisationNameChange < ApplicationRecord
   validate :startdate_must_be_after_last_change
   validate :name_must_be_different_from_current
   validate :startdate_must_be_unique_for_organisation
+  validate :startdate_must_be_before_merge_date
 
   attr_accessor :immediate_change
 
@@ -94,4 +95,13 @@ private
       errors.add(:name, "New name must be different from the current name on the change date.")
     end
   end
+
+  def startdate_must_be_before_merge_date
+    return if startdate.blank? || organisation.merge_date.blank?
+
+    if startdate >= organisation.merge_date
+      errors.add(:startdate, "Start date must be earlier than the organisation's merge date (#{organisation.merge_date.to_formatted_s(:govuk_date)}). You cannot make changes to the name of an organisation after it has merged.") unless immediate_change
+      errors.add(:immediate_change, "Start date must be earlier than the organisation's merge date (#{organisation.merge_date.to_formatted_s(:govuk_date)}). You cannot make changes to the name of an organisation after it has merged.") if immediate_change
+    end
+  end
 end