Fixes #5042 - User merge does not work if both users mentioned the sa…

…me ticket.

Co-authored-by: Florian Liebe <fl@zammad.com>

app/models/user.rb

@@ -731,6 +731,16 @@ def merge(user_id_of_duplicate_user)
     # to prevent any unexpected regressions.)
     User.find(user_id_of_duplicate_user)
 
+    # mentions can not merged easily because the new user could have mentioned
+    # the same ticket so we delete duplicates beforehand
+    Mention.where(user_id: user_id_of_duplicate_user).find_each do |mention|
+      if Mention.exists?(mentionable: mention.mentionable, user_id: id)
+        mention.destroy
+      else
+        mention.update(user_id: id)
+      end
+    end
+
     # merge missing attributes
     Models.merge('User', id, user_id_of_duplicate_user)
 

spec/models/user/merge_spec.rb

@@ -0,0 +1,24 @@
+# Copyright (C) 2012-2024 Zammad Foundation, https://zammad-foundation.org/
+
+require 'rails_helper'
+
+RSpec.describe '.merge', searchindex: true, type: :model do
+  let(:user_1) { create(:agent, groups: Group.all) }
+  let(:user_2) { create(:agent, groups: Group.all) }
+  let(:ticket_1) do
+    ticket = create(:ticket, owner: user_1, group: Group.first)
+    create(:mention, mentionable: ticket, user: user_1)
+    create(:mention, mentionable: ticket, user: user_2)
+    ticket
+  end
+
+  before do
+    user_1
+    user_2
+    ticket_1
+  end
+
+  it 'does merge users' do
+    expect { user_2.merge(user_1.id) }.not_to raise_error
+  end
+end