Move a user's merge requests to the ghost user.

1. When the user is deleted.

2. Refactor out code relating to "migrating records to the ghost user" into a
   `MigrateToGhostUser` concern, which is tested using a shared example.

2 files changed, 41 insertions, 18 deletions

diff --git a/app/services/concerns/users/migrate_to_ghost_user.rb b/app/services/concerns/users/migrate_to_ghost_user.rb
new file mode 100644
index 00000000000..ecbbf3026a0
--- /dev/null
+++ b/app/services/concerns/users/migrate_to_ghost_user.rb
@@ -0,0 +1,38 @@
+# 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::MigrateToGhostUser
+  extend ActiveSupport::Concern
+
+  attr_reader :ghost_user
+
+  def move_associated_records_to_ghost_user(user)
+    # Block the user before moving records to prevent a data race.
+    # For example, if the user creates an issue after `move_issues_to_ghost_user`
+    # runs and before the user is destroyed, the destroy will fail with
+    # an exception.
+    user.block
+
+    user.transaction do
+      @ghost_user = User.ghost
+
+      move_issues_to_ghost_user(user)
+      move_merge_requests_to_ghost_user(user)
+    end
+
+    user.reload
+  end
+
+  private
+
+  def move_issues_to_ghost_user(user)
+    user.issues.update_all(author_id: ghost_user.id)
+  end
+
+  def move_merge_requests_to_ghost_user(user)
+    user.merge_requests.update_all(author_id: ghost_user.id)
+  end
+end
diff --git a/app/services/users/destroy_service.rb b/app/services/users/destroy_service.rb
index a3b32a71a64..e6608e316dc 100644
--- a/app/services/users/destroy_service.rb
+++ b/app/services/users/destroy_service.rb
@@ -1,5 +1,7 @@
 module Users
   class DestroyService
+    include MigrateToGhostUser
+
     attr_accessor :current_user
 
     def initialize(current_user)
@@ -26,7 +28,7 @@ module Users
         ::Projects::DestroyService.new(project, current_user, skip_repo: true).execute
       end
 
-      move_issues_to_ghost_user(user)
+      move_associated_records_to_ghost_user(user)
 
       # Destroy the namespace after destroying the user since certain methods may depend on the namespace existing
       namespace = user.namespace
@@ -35,22 +37,5 @@ module Users
 
       user_data
     end
-
-    private
-
-    def move_issues_to_ghost_user(user)
-      # Block the user before moving issues to prevent a data race.
-      # If the user creates an issue after `move_issues_to_ghost_user`
-      # runs and before the user is destroyed, the destroy will fail with
-      # an exception. We block the user so that issues can't be created
-      # after `move_issues_to_ghost_user` runs and before the destroy happens.
-      user.block
-
-      ghost_user = User.ghost
-
-      user.issues.update_all(author_id: ghost_user.id)
-
-      user.reload
-    end
   end
 end