ImmutableMap<PCollection<?>, FieldAccessDescriptor> getPCollectionFieldAccess() {
    return ImmutableMap.copyOf(pCollectionFieldAccess);
  }
@Test
  public void testFieldAccessNoSchema() {
    Pipeline p = Pipeline.create();
    FieldAccessVisitor fieldAccessVisitor = new FieldAccessVisitor();
    PCollection<String> source = p.apply(Create.of("foo", "bar"));
    source.apply(ParDo.of(new StringDoFn()));

    p.traverseTopologically(fieldAccessVisitor);

    // getAllFields is vacuously true (since there are no fields to access).
    assertTrue(fieldAccessVisitor.getPCollectionFieldAccess().get(source).getAllFields());
  }