Question

I want to define a class method that has access to a local variable. So this would be different for each instance of the class. I know you can make a class method dynamic with lambda like when you use it with named_scope. But can this be done for values that are specific to an instance?

In detail it is the has_attached_file method for the paperclip plugin in rails. I want to pass a lambda for the styles hash so that the image styles can be based off of attributes of the object stored in the DB. Is this possible?

Answer 1

Disclaimer: First, the question (Can you pass self to lambda?) and the problem you’re trying to solve (dynamic styles with paperclip) don’t fully match up. I won’t answer the original question because it’s not entirely related to your problem, and rampion took a valiant stab at it.

I’ll instead answer your paperclip question.

In detail it is the has_attached_file method for the paperclip plugin in rails. I want to pass a lambda for the styles hash so that the image styles can be based off of attributes of the object stored in the DB. Is this possible?

Yes, it is possible. In paperclip, the :styles option can take a Proc. When the attachment is initialized, if a Proc was used, the attachment itself is passed to the Proc. The attachment has a reference to the associated ActiveRecord object, so you can use that to determine your dynamic styles.

For example, your has_attached_file declaration might look something like this (assuming a User and avatar scenario where the user can customize the size of their avatar):

class User < ActiveRecord::Base
  has_attached_file :avatar, :styles => lambda { |attachment| 
    user = attachment.instance
    dimensions = "#{user.avatar_width}x#{user.avatar_height}#"
    { :custom => dimensions }
  }
end