You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: category.rb
    path of file: ./repos/splits-io/app/models
    
    the code of the file until where you have to start completion: class Category < ApplicationRecord
  belongs_to :game

  has_many :runs,      dependent: :null
    
  def merge_into!(category)
    return if category.nil?

    ApplicationRecord.transaction do
      runs.update_all(category_id: category.id)
      rivalries.update_all(category_id: category.id)
      destroy
    end
  end