Adding basic usage (thought-code), not guaranteed to work

lib/dragonfly/active_record_extensions/attachment.rb

@@ -0,0 +1,29 @@
+module Dragonfly
+  module ActiveModelExtensions
+    class Attachment
+      class InvaidGeometryOrStyleNameError < StandardError; end
+
+      def style style_name_or_geometry, format=nil
+        scope = case style_name_or_geometry
+        when Symbol # style_name => :small_rectangle
+          style current_styles[style_name_or_geometry], format
+        when String # geometry => '100x80'
+          job.thumb style_name_or_geometry
+        when Array # geometry + format => ['100x80', :gif]
+          style *style_name_or_geometry
+        when Proc  # Proc for style => lambda { thumb('100x80').process(:greyscale).encode(:png) }
+          job.instance_eval &style_name_or_geometry
+        else
+          raise InvaidGeometryOrStyleNameError
+        end
+        scope = scope.encode(format) if format
+        scope
+      end
+
+      def current_styles
+        parent_model.class.dragonfly_styles_for_attributes[attribute_name.to_sym] || {}
+      end
+
+    end
+  end
+end

lib/dragonfly/active_record_extensions/class_methods.rb

@@ -0,0 +1,24 @@
+module Dragonfly
+  module ActiveModelExtensions
+    module ClassMethods
+      def register_dragonfly_styles macro_name, app
+        (class << self; self; end).class_eval do
+          # Defines e.g. 'image_styles' for any activerecord class body
+          define_method macro_name do |attribute, styles|
+            # Register the new styles
+            dragonfly_styles_for_attributes[attribute] = styles
+          end
+        end
+        app
+      end
+
+      def dragonfly_styles_for_attributes
+        @dragonfly_styles_for_attributes ||= begin
+          parent_class = ancestors.select{|a| a.is_a?(Class) }[1]
+          parent_class.respond_to?(:dragonfly_styles_for_attributes) ? parent_class.dragonfly_styles_for_attributes.dup : {}
+        end
+      end
+
+    end
+  end
+end

lib/dragonfly/app.rb

@@ -0,0 +1,9 @@
+module Dragonfly
+  class App
+    def define_styles_macro mod, macro_name
+      already_extended = (class << mod; self; end).included_modules.include?(ActiveModelExtensions)
+      mod.extend(ActiveModelExtensions) unless already_extended
+      mod.register_dragonfly_styles(macro_name, self)
+    end
+  end
+end