Here's an example that works with basic-sample-network network:
asset SampleAsset identified by assetId {
  o String assetId
  --> SampleParticipant owner
  o String value
  o DateTime timestamp
}
function onSampleTransaction(sampleTransaction) {
  sampleTransaction.asset.value = sampleTransaction.newValue;
  sampleTransaction.asset.timestamp = sampleTransaction.timestamp;
  return getAssetRegistry('org.acme.sample.SampleAsset')
       .then(function (assetRegistry) {
               return assetRegistry.update(sampleTransaction.asset);
        });
}