Map<ProjectionProducer<PTransform<?, ?>>, Map<PCollection<?>, FieldAccessDescriptor>>
      getPushdownOpportunities() {
    return pushdownOpportunities.build();
  }
@Test
  public void testMissingFieldAccessInformation_returnsNoPushdown() {
    Pipeline p = Pipeline.create();
    p.apply(new SimpleSource());

    Map<PCollection<?>, FieldAccessDescriptor> pCollectionFieldAccess = ImmutableMap.of();
    ProjectionProducerVisitor visitor = new ProjectionProducerVisitor(pCollectionFieldAccess);
    p.traverseTopologically(visitor);

    Map<ProjectionProducer<PTransform<?, ?>>, Map<PCollection<?>, FieldAccessDescriptor>>
        pushdownOpportunities = visitor.getPushdownOpportunities();
    Assert.assertTrue(pushdownOpportunities.isEmpty());
  }