Bucket derivatives on disk.

Closes #331.

curation_concerns-models/app/services/curation_concerns/derivative_path.rb

@@ -11,7 +11,12 @@ def derivative_path_for_reference(object, destination_name)
 
         def derivative_path(object, extension, destination_name)
           file_name = destination_name + extension
-          File.join(CurationConcerns.config.derivatives_path, object.id, file_name)
+          File.join(CurationConcerns.config.derivatives_path, pair_path(object.id, file_name))
+        end
+
+        def pair_path(id, file_name)
+          pair = id.split('').each_slice(2).map(&:join).join('/')
+          "#{pair}-#{file_name}"
         end
 
         def extension_for(destination_name)

curation_concerns-models/app/services/curation_concerns/persist_derivatives.rb

@@ -21,10 +21,14 @@ def self.call(object, file, destination_name)
     # file.  It will make the directories in the path if
     # necessary.
     def self.output_file(object, destination_name, &blk)
-      name = DerivativePath.derivative_path_for_reference(object, destination_name)
+      name = derivative_path_factory.derivative_path_for_reference(object, destination_name)
       output_file_dir = File.dirname(name)
       FileUtils.mkdir_p(output_file_dir) unless File.directory?(output_file_dir)
       File.open(name, 'wb', &blk)
     end
+
+    def self.derivative_path_factory
+      DerivativePath
+    end
   end
 end

spec/services/derivative_path_spec.rb

@@ -11,6 +11,6 @@
     let(:object) { double(id: '123') }
     let(:destination_name) { 'thumbnail' }
 
-    it { is_expected.to eq 'tmp/123/thumbnail.jpeg' }
+    it { is_expected.to eq 'tmp/12/3-thumbnail.jpeg' }
   end
 end

spec/services/persist_derivatives_spec.rb

@@ -8,14 +8,14 @@
   describe '.output_file' do
     subject { described_class.output_file(object, destination_name, &block) }
 
-    let(:object) { double(id: '123') }
+    let(:object) { double(id: '1234567') }
     let(:destination_name) { 'thumbnail' }
 
     let(:block) { lambda { true } }
 
     it 'yields to the file' do
-      expect(FileUtils).to receive(:mkdir_p).with('tmp/123')
-      expect(File).to receive(:open).with('tmp/123/thumbnail.jpeg', 'wb') do |*_, &blk|
+      expect(FileUtils).to receive(:mkdir_p).with('tmp/12/34/56')
+      expect(File).to receive(:open).with('tmp/12/34/56/7-thumbnail.jpeg', 'wb') do |*_, &blk|
         expect(blk).to be(block)
       end
       subject