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.

app/services/concerns/users/migrate_to_ghost_user.rb

+# 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

app/services/users/destroy_service.rb

 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

spec/services/users/destroy_spec.rb → spec/services/users/destroy_service_spec.rb

@@ -141,5 +141,21 @@ describe Users::DestroyService, services: true do
         expect(User.exists?(user.id)).to be(false)
       end
     end
+
+    context 'migrating associated records to the ghost user' do
+      context 'issues'  do
+        include_examples "migrating a deleted user's associated records to the ghost user", Issue do
+          let(:created_record) { create(:issue, project: project, author: user) }
+          let(:assigned_record) { create(:issue, project: project, assignee: user) }
+        end
+      end
+
+      context 'merge requests' do
+        include_examples "migrating a deleted user's associated records to the ghost user", MergeRequest do
+          let(:created_record) { create(:merge_request, source_project: project, author: user, target_branch: "first") }
+          let(:assigned_record) { create(:merge_request, source_project: project, assignee: user, target_branch: 'second') }
+        end
+      end
+    end
   end
 end

spec/support/services/user_destroy_service_migrate_to_ghost_user_shared_examples.rb

+require "spec_helper"
+
+shared_examples "migrating a deleted user's associated records to the ghost user" do |record_class|
+  record_class_name = record_class.to_s.titleize.downcase
+
+  let(:project) { create(:project) }
+
+  before do
+    project.add_developer(user)
+  end
+
+  context "for a #{record_class_name} the user has created" do
+    let!(:record) { created_record }
+
+    it "does not delete the #{record_class_name}" do
+      service.execute(user)
+
+      expect(record_class.find_by_id(record.id)).to be_present
+    end
+
+    it "migrates the #{record_class_name} so that the 'Ghost User' is the #{record_class_name} owner" do
+      service.execute(user)
+
+      migrated_record = record_class.find_by_id(record.id)
+
+      expect(migrated_record.author).to eq(User.ghost)
+    end
+
+    it "blocks the user before migrating #{record_class_name}s to the 'Ghost User'" do
+      service.execute(user)
+
+      expect(user).to be_blocked
+    end
+  end
+
+  context "for a #{record_class_name} the user was assigned to" do
+    let!(:record) { assigned_record }
+
+    before do
+      service.execute(user)
+    end
+
+    it "does not delete #{record_class_name}s the user is assigned to" do
+      expect(record_class.find_by_id(record.id)).to be_present
+    end
+
+    it "migrates the #{record_class_name} so that it is 'Unassigned'" do
+      migrated_record = record_class.find_by_id(record.id)
+
+      expect(migrated_record.assignee).to be_nil
+    end
+  end
+end