void run() {
        MutableDocument md;
        md.addField("foo", Value(1));
        ASSERT_EQUALS(1U, md.peek().size());
        ASSERT_EQUALS(1, md.peek()["foo"].getInt());
        md.addField("bar", Value(99));
        ASSERT_EQUALS(2U, md.peek().size());
        ASSERT_EQUALS(99, md.peek()["bar"].getInt());
        // No assertion is triggered by a duplicate field name.
        md.addField("a", Value(5));

        Document final = md.freeze();
        ASSERT_EQUALS(3U, final.size());
        assertRoundTrips(final);
    }
    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();
    }
    Document DocumentSourceProject::getCurrent() {
        Document pInDocument(pSource->getCurrent());

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

        /*
          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.
        */
        pEO->addToDocument(out, pInDocument, /*root=*/pInDocument);

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

            BSONObjBuilder inputBuilder;
            pSource->getCurrent()->toBson(&inputBuilder);
            BSONObj input = inputBuilder.done();

            BSONObjBuilder outputBuilder;
            out.peek().toBson(&outputBuilder);
            BSONObj output = outputBuilder.done();

            BSONObj projected = _simpleProjection.transform(input);

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

        return out.freeze();
    }