user.rb

class User
  def buy_weapon(weapon_type)
    User.transaction do
      weapon = Weapon.create(weapon_type: weapon_type)
      self.decrement!(:money, weapon_type.cost)
      Log.create(message: "User #{id} bought a #{weapon_type.name} for #{weapon_type.cost}.")
    end
  end
end

users_controller.rb

class UsersController < ApplicationController
  def buy_weapon
    weapon_type = WeaponType.find(params[:weapon_type_id])
    current_user.buy_weapon(weapon_type)

    redirect_to dashboard_url
  end
end