Implement review comments from @DouweM for !10467.

1. Have `MigrateToGhostUser` be a service rather than a mixed-in module, to keep
   things explicit. Specs testing the behavior of this class are moved into a
   separate service spec file.

2. Add a `user.reported_abuse_reports` association to make the
   `migrate_abuse_reports` method more consistent with the other `migrate_`
   methods.

1 files changed, 59 insertions, 0 deletions

diff --git a/app/services/users/migrate_to_ghost_user_service.rb b/app/services/users/migrate_to_ghost_user_service.rb
new file mode 100644
index 00000000000..1e1ed1791ec
--- /dev/null
+++ b/app/services/users/migrate_to_ghost_user_service.rb
@@ -0,0 +1,59 @@
+# When a user is destroyed, some of their associated records are
+# moved to a "Ghost User", to prevent these associated records from
+# being destroyed.
+#
+# For example, all the issues/MRs a user has created are _not_ destroyed
+# when the user is destroyed.
+module Users
+  class MigrateToGhostUserService
+    extend ActiveSupport::Concern
+
+    attr_reader :ghost_user, :user
+
+    def initialize(user)
+      @user = user
+    end
+
+    def execute
+      # Block the user before moving records to prevent a data race.
+      # For example, if the user creates an issue after `migrate_issues`
+      # runs and before the user is destroyed, the destroy will fail with
+      # an exception.
+      user.block
+
+      user.transaction do
+        @ghost_user = User.ghost
+
+        migrate_issues
+        migrate_merge_requests
+        migrate_notes
+        migrate_abuse_reports
+        migrate_award_emoji
+      end
+
+      user.reload
+    end
+
+    private
+
+    def migrate_issues
+      user.issues.update_all(author_id: ghost_user.id)
+    end
+
+    def migrate_merge_requests
+      user.merge_requests.update_all(author_id: ghost_user.id)
+    end
+
+    def migrate_notes
+      user.notes.update_all(author_id: ghost_user.id)
+    end
+
+    def migrate_abuse_reports
+      user.reported_abuse_reports.update_all(reporter_id: ghost_user.id)
+    end
+
+    def migrate_award_emoji
+      user.award_emoji.update_all(user_id: ghost_user.id)
+    end
+  end
+end