driver-core/src/main/com/mongodb/BasicDBObjectBuilder.java
public BasicDBObjectBuilder append(final String key, final Object val) {
        _cur().put(key, val);
        return this;
    }
public BasicDBObjectBuilder append(final String key, final Object val) {
        _cur().put(key, val);
        return this;
    }
Appends the key/value to the active object

@param key the field name
@param val the value of the field
@return {@code this} so calls can be chained