1 files changed, 64 insertions, 0 deletions
diff --git a/app/services/topics/merge_service.rb b/app/services/topics/merge_service.rb
new file mode 100644
index 00000000000..0d256579fe0
--- /dev/null
+++ b/app/services/topics/merge_service.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+module Topics
+  class MergeService
+    attr_accessor :source_topic, :target_topic
+
+    def initialize(source_topic, target_topic)
+      @source_topic = source_topic
+      @target_topic = target_topic
+    end
+
+    def execute
+      validate_parameters!
+
+      ::Projects::ProjectTopic.transaction do
+        move_project_topics
+        refresh_target_topic_counters
+        delete_source_topic
+      end
+    end
+
+    private
+
+    def validate_parameters!
+      raise ArgumentError, 'The source topic is not a topic.' unless source_topic.is_a?(Projects::Topic)
+      raise ArgumentError, 'The target topic is not a topic.' unless target_topic.is_a?(Projects::Topic)
+      raise ArgumentError, 'The source topic and the target topic are identical.' if source_topic == target_topic
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def move_project_topics
+      project_ids_for_projects_currently_using_source_and_target = ::Projects::ProjectTopic
+        .where(topic_id: target_topic).select(:project_id)
+      # Only update for projects that exclusively use the source topic
+      ::Projects::ProjectTopic.where(topic_id: source_topic.id)
+        .where.not(project_id: project_ids_for_projects_currently_using_source_and_target)
+        .update_all(topic_id: target_topic.id)
+
+      # Delete source topic for projects that were using source and target
+      ::Projects::ProjectTopic.where(topic_id: source_topic.id).delete_all
+    end
+
+    def refresh_target_topic_counters
+      target_topic.update!(
+        total_projects_count: total_projects_count(target_topic.id),
+        non_private_projects_count: non_private_projects_count(target_topic.id)
+      )
+    end
+
+    def delete_source_topic
+      source_topic.destroy!
+    end
+
+    def total_projects_count(topic_id)
+      ::Projects::ProjectTopic.where(topic_id: topic_id).count
+    end
+
+    def non_private_projects_count(topic_id)
+      ::Projects::ProjectTopic.joins(:project).where(topic_id: topic_id).where('projects.visibility_level in (10, 20)')
+        .count
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+  end
+end