00_daemon.rb

frequency = 50/1000 # 50 ms

loop do
  moving_objects = Ship.all + Projectile.all

  moving_objects.each do |moving_object|
    moving_object.move
    moving_object.collisions.each do |collision|
      collision.object.collides_with(collision.other_object)
    end
  end

  sleep(frequency)
end

10_moving_object.rb

class MovingObject < ActiveRecord::Base
  def move
    # Move the object.
  end


  def collisions
    # Find things that collides with me.
  end
end

20_ship.rb

class Ship < MovingObject
  def collides_with(other_object)
    if other_object.is_a(Ship)
      explode
      other_object.explode
    end
  end


  def receive_damage(damage)
    current_health -= damage
    explode if current_health == 0
  end


  def explode
    destroy
  end
end

30_projectile.rb

class Projectile < MovingObject
  def collides_with(other_object)
    if other_object.is_a?(Ship)
      damage_ship(other_object)
    elsif other_object.is_a?(Projectile)
      destroy_projectile(other_object)
    end
  end


  def damage_ship(ship)
    final_damage = power - ship.armor
    ship.receive_damage(final_damage)
  end


  def destroy_projectile(projectile)
    projectile.explode
  end


  def explode
    destroy
  end
end