Ruby on Rails - Paperclip and dynamic parameters

Question

I'm writing some image upload code for Ruby on Rails with Paperclip, and I've got a working solution but it's very hacky so I'd really appreciate advice on how to better implement it. I have an 'Asset' class containing information about the uploaded images including the Paperclip attachment, and a 'Generator' class that encapsulates sizing information. Each 'Project' has multiple assets and generators; all Assets should be resized according to the sizes specified by each generator; each Project therefore has a certain set of sizes that all of its assets should have.

Generator model:

class Generator < ActiveRecord::Base
  attr_accessible :height, :width

  belongs_to :project

  def sym
    "#{self.width}x#{self.height}".to_sym
  end
end

Asset model:

class Asset < ActiveRecord::Base
  attr_accessible :filename,
    :image # etc.
  attr_accessor :generators

  has_attached_file :image,
    :styles => lambda { |a| a.instance.styles }

  belongs_to :project

  # this is utterly horrendous
  def styles
    s = {}
    if @generators == nil
      @generators = self.project.generators
    end

    @generators.each do |g|
      s[g.sym] = "#{g.width}x#{g.height}"
    end
    s
  end
end

Asset controller create method:

  def create
    @project = Project.find(params[:project_id])
    @asset = Asset.new
    @asset.generators = @project.generators
    @asset.update_attributes(params[:asset])
    @asset.project = @project
    @asset.uploaded_by = current_user

    respond_to do |format|
      if @asset.save_(current_user)
        @project.last_asset = @asset
        @project.save

        format.html { redirect_to project_asset_url(@asset.project, @asset), notice: 'Asset was successfully created.' }
        format.json { render json: @asset, status: :created, location: @asset }
      else
        format.html { render action: "new" }
        format.json { render json: @asset.errors, status: :unprocessable_entity }
      end
    end
  end

The problem I am having is a chicken-egg issue: the newly created Asset doesn't know which generators (size specifications) to use until after it's been instantiated properly. I tried using @project.assets.build, but then the Paperclip code is still executed before the Asset gets its project association set and nils out on me.

The 'if @generators == nil' hack is so the update method will work without further hacking in the controller.

All in all it feels pretty bad. Can anyone suggest how to write this in a more sensible way, or even an approach to take for this kind of thing?

Thanks in advance! :)

Answer 1

I ran into the same Paperclip chicken/egg issue on a project trying to use dynamic styles based on the associated model with a polymorphic relationship. I've adapted my solution to your existing code. An explanation follows:

class Asset < ActiveRecord::Base
  attr_accessible :image, :deferred_image
  attr_writer :deferred_image

  has_attached_file :image,
    :styles => lambda { |a| a.instance.styles }

  belongs_to :project

  after_save :assign_deferred_image

  def styles
    project.generators.each_with_object({}) { |g, hsh| hsh[g.sym] = "#{g.width}x#{g.height}" }
  end

  private
  def assign_deferred_image
    if @deferred_image
      self.image = @deferred_image
      @deferred_image = nil
      save!
    end
  end
end

Basically, to get around the issue of Paperclip trying to retrieve the dynamic styles before the project relation information has been propagated, you can assign all of the image attributes to a non-Paperclip attribute (in this instance, I have name it deferred_image). The after_save hook assigns the value of @deferred_image to self.image, which kicks off all the Paperclip jazz.

Your controller becomes:

# AssetsController
def create
  @project = Project.find(params[:project_id])
  @asset = @project.assets.build(params[:asset])
  @asset.uploaded_by = current_user

  respond_to do |format|
    # all this is unrelated and can stay the same
  end
end

And the view:

<%= form_for @asset do |f| %>
  <%# other asset attributes %>
  <%= f.label :deferred_upload %>
  <%= f.file_field :deferred_upload %>
  <%= f.submit %>
<% end %>

This solution also allows using accepts_nested_attributes for the assets relation in the Project model (which is currently how I'm using it - to upload assets as part of creating/editing a Project).

There are some downsides to this approach (ex. validating the Paperclip image in relation to the validity of the Asset instance gets tricky), but it's the best I could come up with short of monkey patching Paperclip to somehow defer execution of the style method until after the association information had been populated.

I'll be keeping an eye on this question to see if anyone has a better solution to this problem!

---

At the very least, if you choose to keep using your same solution, you can make the following stylistic improvement to your Asset#styles method:

def styles
  (@generators || project.generators).each_with_object({}) { |g, hsh| hsh[g.sym] = "#{g.width}x#{g.height}" }
end

Does the exact same thing as your existing method, but more succinctly.