driver-core/src/main/com/mongodb/BasicDBObject.java
@Override
    public BasicDBObject append(final String key, final Object val) {
        put(key, val);
        return this;
    }
@Override
    public BasicDBObject append(final String key, final Object val) {
        put(key, val);
        return this;
    }
Add a key/value pair to this object

@param key the field name
@param val the field value
@return this BasicDBObject with the new values added