Enable conditional `pattern` assignment to Interpolations

lib/paperclip/interpolations.rb

@@ -25,7 +25,9 @@ module Paperclip
 
     # Perform the actual interpolation. Takes the pattern to interpolate
     # and the arguments to pass, which are the attachment and style name.
+    # You can pass a :symbol as pattern assigning some method in your model class.
     def self.interpolate pattern, *args
+      pattern = args.first.instance.send(pattern) if pattern.kind_of? Symbol
       all.reverse.inject( pattern.dup ) do |result, tag|
         result.gsub(/:#{tag}/) do |match|
           send( tag, *args )

test/attachment_test.rb

@@ -36,6 +36,13 @@ class AttachmentTest < Test::Unit::TestCase
     assert_equal "fake_models/blegga.png", @attachment.url
   end
 
+  should "return the url by executing and interpolating the default_url when assigned with symbol as method in attachment model" do
+    @attachment = attachment :default_url => :some_method_to_determine_default_url
+    @model = @attachment.instance
+    @model.stubs(:some_method_to_determine_default_url).returns(":class/female_:style_blegga.png")
+    assert_equal "fake_models/female_foostyle_blegga.png", @attachment.url(:foostyle)
+  end
+
   context "Attachment default_options" do
     setup do
       rebuild_model