Current ObjectMapper configuration doesn't serialise null fields. The test data (artists.dat) contains empty fields that cause CascadingHadoopTest.testWriteToES() to fail with:

org.codehaus.jackson.map.JsonMappingException: No serializer found for class org.apache.hadoop.io.NullWritable

To make it work with the existing OM configuration the test needs to filter out all null-field entries before writing the index: fa62b72

pipe = new Each(pipe, 
   new Fields("id", "name", "url", "picture"), new FilterNull());

Related issues: #27, #30