MongoDB BSON codec not being used while encoding object

After several days of research, I've figured out a solution.

The DutyBlockCodec depends on the LocalDateCodec (which I created) in order to encode/decode. This dependency isn't satisfied just by adding the two codecs into the same codec registry. The solution is to pass a CodecRegistry object containing the codecs that DutyBlockCodec depends on (e.g. a CodecRegistry containing within it the LocalDateCodec) to the DutyBlockCodec's constructor, which is stored as a member variable. In order to use the LocalDateCodec to encode, I use the EncoderContext.encodeWithChildContext() method, passing in the codec, writer, and element to encode. Additionally, I write individual fields rather than writing a Document as a String (as in my original code). Thus the DutyBlock codec ends up looking like this:

public class DutyBlockCodec implements Codec<DutyBlock> {
    private final CodecRegistry codecRegistry;

    public DutyBlockCodec(final CodecRegistry codecRegistry) {
        this.codecRegistry = codecRegistry;
    }

    @Override
    public void encode(BsonWriter writer, DutyBlock t, EncoderContext ec) {
        writer.writeStartDocument();
            Codec dateCodec = codecRegistry.get(LocalDate.class);
            writer.writeName("startDate");
            ec.encodeWithChildContext(dateCodec, writer, t.getStartDate());
            writer.writeName("endDate");
            ec.encodeWithChildContext(dateCodec, writer, t.getEndDate());
            writer.writeName("blockLength");
            writer.writeInt32(t.getBlockLength());
            writer.writeName("pointValue");
            writer.writeDouble(t.getPointValue());

            //Writing ArrayList of RAs
            writer.writeName("assigned");
            writer.writeStartArray();
                for (Ra ra : t.getRasOnDuty()) {
                    Codec raCodec = codecRegistry.get(Ra.class);
                    ec.encodeWithChildContext(raCodec, writer, ra);
                }
            writer.writeEndArray();
        writer.writeEndDocument();
    }

    @Override
    public Class<DutyBlock> getEncoderClass() {
        return DutyBlock.class;
    }

    @Override
    public DutyBlock decode(BsonReader reader, DecoderContext dc) {
        reader.readStartDocument();
            Codec<LocalDate> dateCodec = codecRegistry.get(LocalDate.class);
            reader.readName();
            LocalDate startDate = dateCodec.decode(reader, dc);
            reader.readName();
            LocalDate endDate = dateCodec.decode(reader, dc);
            reader.readName();
            int blockLength = reader.readInt32();
            reader.readName();
            double pointValue = reader.readDouble();

            //Reading ArrayList of RAs
            reader.readName();
            Codec<Ra> raCodec = codecRegistry.get(Ra.class);
            ArrayList<Ra> rasOnDuty = new ArrayList<>();
            reader.readStartArray();
                while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
                    rasOnDuty.add(raCodec.decode(reader, dc));
                }
            reader.readEndArray();
        reader.readEndDocument();

        return new DutyBlock(startDate, endDate, blockLength, pointValue, rasOnDuty);
    }

}

DutyBlockCodec depends on another codec, and so requires a CodecRegistry to be passed in on its constructor. While I believe it is possible to create a CodecRegistry with the LocalDateCodec, then pass this as an argument to DutyBlockCodec's constructor, then create another CodecRegistry containing both LocalDateCodec and DutyBlockCodec, this is rather confusing, and MongoDB provides a functionality, the CodecProvider to facilitate this process.

Using the CodecProvider interface, I wrote a DutyBlockCodecProvider

public class DutyBlockCodecProvider implements CodecProvider {
    @Override
    public <T> Codec<T> get(Class<T> type, CodecRegistry cr) {
        if (type == DutyBlock.class) {
            return (Codec<T>) new DutyBlockCodec(cr);
        }
        return null;
    }
}

I added these CodecProviders to the MongoDB Client using the CodecRegistries.fromProviders() method.

CodecRegistry codecRegistry = CodecRegistries.fromRegistries(
            CodecRegistries.fromCodecs(new LocalDateCodec()),
            CodecRegistries.fromProviders(
                    new RaCodecProvider(),
                    new DutyBlockCodecProvider(),
                    new ScheduledDutyCodecProvider()),
            MongoClient.getDefaultCodecRegistry());  
    MongoClientOptions options = MongoClientOptions.builder()
            .codecRegistry(codecRegistry).build();
    mongoClient = new MongoClient(new ServerAddress(), options);
    db = mongoClient.getDatabase("DutySchedulerDB");

My source code for this project can be found at https://github.com/desrepair/DutyScheduler I'm open to answering any questions people may have.