Hao's Thoughts

Ruby, Rails, Objective-C and everything else

Polymorphic Image Styles With Paperclip

You may have ran into the following scenario that your rails app having 1+ domain models having image with them, say Album has one cover, User has one avatar:

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class Image < ActiveRecord::Base
  belongs_to :viewable, :polymorphic => true

  validate :viewable, :presence => true

  has_attached_file :attachment
end

class User < ActiveRecord::Base
  has_one :avatar, :class_name => "Image", :as => :viewable, :dependent => :destroy
end

class Album < ActiveRecord::Base
  has_one :cover, :class_name => "Image", :as => :viewable, :dependent => :destroy
end

As you can see, an image belongs to a viewable object, the viweable object could be an instance of User or Album or anything else.

The presumption here is that the cover is having a set of styles different from with the avatar, but we only have one Image class, what are we supposed to do? We dispatch. We let the viewable object decide the styles of the image.

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class Image < ActiveRecord::Base
  belongs_to :viewable, :polymorphic => true

  validate :viewable, :presence => true

  has_attached_file :attachment,
    :styles => Proc.new {|a| a.instance.viewable.image_styles}
end

class User < ActiveRecord::Base
  has_one :avatar, :class_name => "Image", :as => :viewable, :dependent => :destroy

  def image_styles
    {
      :thumb => "30x30>",
      :medium => "100x100>"
    }
  end
end

class Album < ActiveRecord::Base
  has_one :cover, :class_name => "Image", :as => :viewable, :dependent => :destroy

  def image_styles
    {
      :cart => "50x50>"
      :large => "300x300>"
    }
  end
end

We now have one Image class having different styles for different viewable objects, we can also do the same thing to default_url and path. Leave me a comment if you have better solution, thanks a lot.

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