boost::optional<Document> DocumentSourceRedact::redactObject() {
    const Value expressionResult = _expression->evaluate(_variables.get());

    if (expressionResult == keepVal) {
        return _variables->getDocument(_currentId);
    } else if (expressionResult == pruneVal) {
        return boost::optional<Document>();
    } else if (expressionResult == descendVal) {
        const Document in = _variables->getDocument(_currentId);
        MutableDocument out;
        out.copyMetaDataFrom(in);
        FieldIterator fields(in);
        while (fields.more()) {
            const Document::FieldPair field(fields.next());

            // This changes CURRENT so don't read from _variables after this
            const Value val = redactValue(field.second);
            if (!val.missing()) {
                out.addField(field.first, val);
            }
        }
        return out.freeze();
    } else {
        uasserted(17053,
                  str::stream() << "$redact's expression should not return anything "
                                << "aside from the variables $$KEEP, $$DESCEND, and "
                                << "$$PRUNE, but returned "
                                << expressionResult.toString());
    }
}
    boost::optional<Document> DocumentSourceProject::getNext() {
        pExpCtx->checkForInterrupt();

        boost::optional<Document> input = pSource->getNext();
        if (!input)
            return boost::none;

        /* create the result document */
        const size_t sizeHint = pEO->getSizeHint();
        MutableDocument out (sizeHint);
        out.copyMetaDataFrom(*input);

        /*
          Use the ExpressionObject to create the base result.

          If we're excluding fields at the top level, leave out the _id if
          it is found, because we took care of it above.
        */
        _variables->setRoot(*input);
        pEO->addToDocument(out, *input, _variables.get());
        _variables->clearRoot();

#if defined(_DEBUG)
        if (!_simpleProjection.getSpec().isEmpty()) {
            // Make sure we return the same results as Projection class

            BSONObj inputBson = input->toBson();
            BSONObj outputBson = out.peek().toBson();

            BSONObj projected = _simpleProjection.transform(inputBson);

            if (projected != outputBson) {
                log() << "$project applied incorrectly: " << getRaw() << endl;
                log() << "input:  " << inputBson << endl;
                log() << "out: " << outputBson << endl;
                log() << "projected: " << projected << endl;
                verify(false); // exits in _DEBUG builds
            }
        }
#endif

        return out.freeze();
    }
    boost::optional<Document> DocumentSourceProject::getNext() {
        pExpCtx->checkForInterrupt();

        boost::optional<Document> input = pSource->getNext();
        if (!input)
            return boost::none;

        /* create the result document */
        const size_t sizeHint = pEO->getSizeHint();
        MutableDocument out (sizeHint);
        out.copyMetaDataFrom(*input);

        /*
          Use the ExpressionObject to create the base result.

          If we're excluding fields at the top level, leave out the _id if
          it is found, because we took care of it above.
        */
        _variables->setRoot(*input);
        pEO->addToDocument(out, *input, _variables.get());
        _variables->clearRoot();

        return out.freeze();
    }