Migrate issues authored by deleted user to the Ghost user

changelogs/unreleased/36860-migrate-issues-author.yml

+---
+title: Migrate issues authored by deleted user to the Ghost user
+merge_request:
+author:
+type: fixed

db/migrate/20170825104051_migrate_issues_to_ghost_user.rb

+class MigrateIssuesToGhostUser < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class User < ActiveRecord::Base
+    self.table_name = 'users'
+  end
+
+  class Issue < ActiveRecord::Base
+    self.table_name = 'issues'
+
+    include ::EachBatch
+  end
+
+  def reset_column_in_migration_models
+    ActiveRecord::Base.clear_cache!
+
+    ::User.reset_column_information
+  end
+
+  def up
+    reset_column_in_migration_models
+
+    # we use the model method because rewriting it is too complicated and would require copying multiple methods
+    ghost_id = ::User.ghost.id
+
+    Issue.where('NOT EXISTS (?)', User.unscoped.select(1).where('issues.author_id = users.id')).each_batch do |relation|
+      relation.update_all(author_id: ghost_id)
+    end
+  end
+
+  def down
+  end
+end

db/migrate/20170901071411_add_foreign_key_to_issue_author.rb

+class AddForeignKeyToIssueAuthor < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_foreign_key(:issues, :users, column: :author_id, on_delete: :nullify)
+  end
+
+  def down
+    remove_foreign_key(:issues, column: :author_id)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170824162758) do
+ActiveRecord::Schema.define(version: 20170901071411) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1707,6 +1707,7 @@ ActiveRecord::Schema.define(version: 20170824162758) do
   add_foreign_key "issue_assignees", "users", name: "fk_5e0c8d9154", on_delete: :cascade
   add_foreign_key "issue_metrics", "issues", on_delete: :cascade
   add_foreign_key "issues", "projects", name: "fk_899c8f3231", on_delete: :cascade
+  add_foreign_key "issues", "users", column: "author_id", name: "fk_05f1e72feb", on_delete: :cascade
   add_foreign_key "label_priorities", "labels", on_delete: :cascade
   add_foreign_key "label_priorities", "projects", on_delete: :cascade
   add_foreign_key "labels", "namespaces", column: "group_id", on_delete: :cascade

spec/migrations/migrate_issues_to_ghost_user_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20170825104051_migrate_issues_to_ghost_user.rb')
+
+describe MigrateIssuesToGhostUser, :migration do
+  describe '#up' do
+    let(:projects) { table(:projects) }
+    let(:issues) { table(:issues) }
+    let(:users) { table(:users) }
+
+    before do
+      projects.create!(name: 'gitlab')
+      user = users.create(email: 'test@example.com')
+      issues.create(title: 'Issue 1', author_id: nil, project_id: 1)
+      issues.create(title: 'Issue 2', author_id: user.id, project_id: 1)
+    end
+
+    context 'when ghost user exists' do
+      let!(:ghost) { users.create(ghost: true, email: 'ghost@example.com') }
+
+      it 'does not create a new user' do
+        expect { schema_migrate_up! }.not_to change { User.count }
+      end
+
+      it 'migrates issues where author = nil to the ghost user' do
+        schema_migrate_up!
+
+        expect(issues.first.reload.author_id).to eq(ghost.id)
+      end
+
+      it 'does not change issues authored by an existing user' do
+        expect { schema_migrate_up! }.not_to change { issues.second.reload.author_id}
+      end
+    end
+
+    context 'when ghost user does not exist' do
+      it 'creates a new user' do
+        expect { schema_migrate_up! }.to change { User.count }.by(1)
+      end
+
+      it 'migrates issues where author = nil to the ghost user' do
+        schema_migrate_up!
+
+        expect(issues.first.reload.author_id).to eq(User.ghost.id)
+      end
+
+      it 'does not change issues authored by an existing user' do
+        expect { schema_migrate_up! }.not_to change { issues.second.reload.author_id}
+      end
+    end
+  end
+end