Add a #to_param interpolation

lib/paperclip/interpolations.rb

@@ -99,6 +99,11 @@ def id attachment, style_name
       attachment.instance.id
     end
 
+    # Returns the #to_param of the instance.
+    def param attachment, style_name
+      attachment.instance.to_param
+    end
+
     # Returns the fingerprint of the instance.
     def fingerprint attachment, style_name
       attachment.fingerprint

test/integration_test.rb

@@ -34,20 +34,20 @@ class IntegrationTest < Test::Unit::TestCase
     should "create its thumbnails properly" do
       assert_match /\b50x50\b/, `identify "#{@dummy.avatar.path(:thumb)}"`
     end
-    
+
     context 'reprocessing with unreadable original' do
       setup { File.chmod(0000, @dummy.avatar.path) }
-      
+
       should "not raise an error" do
         assert_nothing_raised do
           @dummy.avatar.reprocess!
         end
       end
-      
+
       should "return false" do
         assert ! @dummy.avatar.reprocess!
       end
-      
+
       teardown { File.chmod(0644, @dummy.avatar.path) }
     end
 
@@ -467,6 +467,18 @@ def s3_headers_for attachment, style
         @files_on_s3 = s3_files_for @dummy.avatar
       end
 
+      context 'assigning itself to a new model' do
+        setup do
+          @d2 = Dummy.new
+          @d2.avatar = @dummy.avatar
+          @d2.save
+        end
+
+        should "have the same name as the old file" do
+          assert_equal @d2.avatar.original_filename, @dummy.avatar.original_filename
+        end
+      end
+
       should "have the same contents as the original" do
         @file.rewind
         assert_equal @file.read, @files_on_s3[:original].read

test/interpolations_test.rb

@@ -50,6 +50,13 @@ class InterpolationsTest < Test::Unit::TestCase
     assert_equal "png", Paperclip::Interpolations.extension(attachment, :style)
   end
 
+  should "return the #to_param of the attachment" do
+    attachment = mock
+    attachment.expects(:to_param).returns("23-awesome")
+    attachment.expects(:instance).returns(attachment)
+    assert_equal "23-awesome", Paperclip::Interpolations.param(attachment, :style)
+  end
+
   should "return the id of the attachment" do
     attachment = mock
     attachment.expects(:id).returns(23)