return metadata identifiers in preferred order

app/services/metadata_service.rb

@@ -16,9 +16,10 @@ def can_resolve?(identifier)
       handlers.key?(prefix.to_sym)
     end
 
-    # TODO: Return a prioritized list
+    # return the identfiers found in the same order of the known prefixes we specified
     def resolvable(identifiers)
-      identifiers.select { |identifier| can_resolve?(identifier) }
+      res_ids = identifiers.select { |identifier| can_resolve?(identifier) }
+      MetadataService.known_prefixes.map { |prefix| res_ids.find { |res_id| res_id.start_with?(prefix.to_s) } }.compact
     end
 
     def fetch(identifier)

spec/services/metadata_service_spec.rb

@@ -7,6 +7,20 @@
     expect { described_class.fetch('foo:bar') }.to raise_exception(MetadataError)
   end
 
+  describe 'resolvable' do
+    it 'returns only resolvable identifiers' do
+      expect(described_class.resolvable(['bogus:1234', 'catkey:5678'])).to eq(['catkey:5678'])
+    end
+
+    it 'returns only resolvable identifiers in preferred order when more one is resolvable' do
+      expect(described_class.resolvable(['barcode:1234', 'catkey:5678'])).to eq(['catkey:5678', 'barcode:1234'])
+    end
+
+    it 'returns an empty array when no resolvable identifiers are found' do
+      expect(described_class.resolvable(['bogus:1234', 'nada:5678'])).to eq([])
+    end
+  end
+
   describe 'Symphony handler' do
     let(:resource) { instance_double(MarcxmlResource, mods: mods) }
     let(:mods) { File.read(File.join(fixture_dir, 'mods_record.xml')) }