public <T> JoinBuilder<Q, K,T> join(Path<T> ref, Path<T> target) {
        return new JoinBuilder<Q, K,T>(queryMixin, ref, target);
    }
Define a join

@param ref reference
@param target join target
@return join builder
querydsl-mongodb/src/main/java/com/querydsl/mongodb/AbstractMongodbQuery.java