Dynamic generate bucket name for S3 - ruby-on-rails-3

Question

Im trying to create an dynamic bucket name depending on my polymorphic association type.
My first approach was trying something like this:
class PostImage < ActiveRecord::Base
belongs_to :imageable, :polymorphic => true
has_attached_file :image, :styles => { :small => "200x200>", :thumb => "50x50>" },
:storage => :s3,
:s3_credentials => "#{Rails.root}/config/s3.yml",
:path => "/:style/:id/:filename",
:bucket => self.imageable_type.to_s
end
If i try to create a new object i got the next error:
NoMethodError: undefined method `imageable_type' for #< Class:0x007fd3fe0b15d8
I find out on the S3 documentation this:
bucket: This is the name of the S3 bucket that will store your files. Remember that the bucket must be unique across all of Amazon S3. If the bucket does not exist Paperclip will attempt to create it. The bucket name will not be interpolated. You can define the bucket as a Proc if you want to determine it's name at runtime. Paperclip will call that Proc with attachment as the only argument.
The problem is that i don't get how i can get this working to set the name of my polymorphic association as the name of the bucket.
Any help will be appreciated.

Answer 1

Hope it help somebody,
The final solution was based on this post: rails paperclip S3 with dynamic bucket name
Read the post for get a better explanation of how use the Proc.
The final code:
class PostImage < ActiveRecord::Base
belongs_to :imageable, :polymorphic => true
has_attached_file :image, :styles => {
:square=> "170x170#",
:rectangle=> "220x170#",
:large => "600x400#" }, :storage => :s3, :s3_credentials => "#{Rails.root}/config/s3.yml",
:path => "/:style/:id/:filename",
:bucket => lambda { |attachment| "#{attachment.instance.imageable_type.to_s.downcase}-gallery" }
end