```ruby
class CreateCustomVersions < ActiveRecord::Migration[6.1]
  def change
    create_table :custom_versions do |t|
      t.string :item_type, null: false
      t.integer :item_id, null: false
      t.string :event, null: false
      t.string :whodunnit
      t.text :object
      t.datetime :created_at
    end
    add_index :custom_versions, [:item_type, :item_id]
  end
end
```

Update migration for compatibility with older Rails versions
user
```ruby
class CreateCustomVersions < ActiveRecord::Migration[5.0]
  def change
    create_table :custom_versions do |t|
      t.string :item_type, null: false
      t.integer :item_id, null: false
      t.string :event, null: false
      t.string :whodunnit
      t.text :object
      t.datetime :created_at
    end
    add_index :custom_versions, [:item_type, :item_id]
  end
end
```
assistant