1 files changed, 58 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/remove_duplicate_services.rb b/lib/gitlab/background_migration/remove_duplicate_services.rb
new file mode 100644
index 00000000000..59fb9143a72
--- /dev/null
+++ b/lib/gitlab/background_migration/remove_duplicate_services.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Remove duplicated service records with the same project and type.
+    # These were created in the past for unknown reasons, and should be blocked
+    # now by the uniqueness validation in the Service model.
+    class RemoveDuplicateServices
+      # See app/models/service
+      class Service < ActiveRecord::Base
+        include EachBatch
+
+        self.table_name = 'services'
+        self.inheritance_column = :_type_disabled
+
+        scope :project_ids_with_duplicates, -> do
+          select(:project_id)
+            .distinct
+            .where.not(project_id: nil)
+            .group(:project_id, :type)
+            .having('count(*) > 1')
+        end
+
+        scope :types_with_duplicates, -> (project_ids) do
+          select(:project_id, :type)
+            .where(project_id: project_ids)
+            .group(:project_id, :type)
+            .having('count(*) > 1')
+        end
+      end
+
+      def perform(*project_ids)
+        types_with_duplicates = Service.types_with_duplicates(project_ids).pluck(:project_id, :type)
+
+        types_with_duplicates.each do |project_id, type|
+          remove_duplicates(project_id, type)
+        end
+      end
+
+      private
+
+      def remove_duplicates(project_id, type)
+        scope = Service.where(project_id: project_id, type: type)
+
+        # Build a subquery to determine which service record is actually in use,
+        # by querying for it without specifying an order.
+        #
+        # This should match the record returned by `Project#find_service`,
+        # and the `has_one` service associations on `Project`.
+        correct_service = scope.select(:id).limit(1)
+
+        # Delete all other services with the same `project_id` and `type`
+        duplicate_services = scope.where.not(id: correct_service)
+        duplicate_services.delete_all
+      end
+    end
+  end
+end