Added support for S3 bucket being defined as a Proc and therefore determined at runtime.

(cherry picked from commit 74de215ea28628af94b2c4a0d1b005395247fdc1)

lib/paperclip/storage.rb

@@ -106,6 +106,8 @@ module Paperclip
     # * +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.
     # * +url+: There are two options for the S3 url. You can choose to have the bucket's name
     #   placed domain-style (bucket.s3.amazonaws.com) or path-style (s3.amazonaws.com/bucket).
     #   Normally, this won't matter in the slightest and you can leave the default (which is
@@ -122,6 +124,7 @@ module Paperclip
         base.instance_eval do
           @s3_credentials = parse_credentials(@options[:s3_credentials])
           @bucket         = @options[:bucket]         || @s3_credentials[:bucket]
+          @bucket         = @bucket.call(self) if @bucket.is_a?(Proc)
           @s3_options     = @options[:s3_options]     || {}
           @s3_permissions = @options[:s3_permissions] || 'public-read'
           @s3_protocol    = @options[:s3_protocol]    || (@s3_permissions == 'public-read' ? 'http' : 'https')

test/storage_test.rb

@@ -126,6 +126,19 @@ class StorageTest < Test::Unit::TestCase
       end
     end
   end
+  
+  context "An attachment with S3 storage and bucket defined as a Proc" do
+    setup do
+      rebuild_model :storage => :s3,
+                    :bucket => lambda { |attachment| "bucket_#{attachment.instance.other}" },
+                    :s3_credentials => {:not => :important}
+    end
+    
+    should "get the right bucket name" do
+      assert "bucket_a", Dummy.new(:other => 'a').avatar.bucket_name
+      assert "bucket_b", Dummy.new(:other => 'b').avatar.bucket_name
+    end
+  end
 
   context "An attachment with S3 storage and specific s3 headers set" do
     setup do