@Test
public void attachDocuments_EMPTY_DOCS() {

    
    
    // given
    
    JavaPairRDD<String, String> matchedCitations = sparkContext.parallelizePairs(ImmutableList.of(
            new Tuple2<>(citation1.id(), document1.id()),
            new Tuple2<>(citation1.id(), document2.id()),
            new Tuple2<>(citation4.id(), document5.id())));
    
    JavaPairRDD<String, MatchableEntity> documents = sparkContext.parallelizePairs(Lists.newArrayList());
    
    
    
    // execute
    
    JavaPairRDD<String, MatchableEntity> actualCitIdDocPairs = documentAttacher.attachDocuments(matchedCitations, documents);
    
    
    // assert
    
    assertEquals(actualCitIdDocPairs.count(), 0);
    
}