Ignore obsolete columns for resource_milestone_events

app/models/resource_milestone_event.rb

 # frozen_string_literal: true
 
 class ResourceMilestoneEvent < ResourceEvent
+  include IgnorableColumns
+
   belongs_to :issue
   belongs_to :merge_request
   belongs_to :milestone
@@ -18,6 +20,8 @@ class ResourceMilestoneEvent < ResourceEvent
   # state is used for issue and merge request states.
   enum state: Issue.available_states.merge(MergeRequest.available_states)
 
+  ignore_columns %i[reference reference_html cached_markdown_version], remove_with: '13.1', remove_after: '2020-06-22'
+
   def self.issuable_attrs
     %i(issue merge_request).freeze
   end

app/services/issuable/clone/attributes_rewriter.rb

@@ -74,7 +74,7 @@ module Issuable
 
           if matching_destination_milestone.present?
             event.attributes
-              .except('id', 'reference', 'reference_html')
+              .except('id')
               .merge(entity_key => new_entity.id,
                      'milestone_id' => matching_destination_milestone.id,
                      'action' => ResourceMilestoneEvent.actions[event.action],

spec/services/issuable/clone/attributes_rewriter_spec.rb

@@ -112,9 +112,6 @@ describe Issuable::Clone::AttributesRewriter do
         expect(event.milestone_id).to eq(expected_attrs[:milestone].id)
         expect(event.action).to eq(expected_attrs[:action])
         expect(event.state).to eq(expected_attrs[:state])
-
-        expect(event.reference).to be_nil
-        expect(event.reference_html).to be_nil
       end
     end
   end