code: public static Bson convertToIndex(final Constraints.ConstraintHost fields) {
      final Document document = new Document();
      fields.accept(new ConvertToIndex(document));
      return document;
    }
docs: Builds bson with index definition. The difference with
  {@link #convertToBson(Constraints.ConstraintHost)} is that this method fails on duplicate
  fields. Currently only one index is allowed per field.
  @see <a href="https://docs.mongodb.com/manual/indexes/">Mongo Indexes</a>