JPA 标准 API 按 NULL 最后排序

我使用 JPA 标准 API 从数据库中获取记录。 我有实体Record带字段dateTime可以为空。我会编码：

public List<Record> find(RecordFilter recordFilter, int page, int pageSize) {
    CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
    CriteriaQuery<Record> criteriaQuery = criteriaBuilder.createQuery(Record.class);
    Root<Record> recordRoot = criteriaQuery.from(Record.class);

    /*
     * JOINS. Left Joins are used for optional fields, or fields inside of the optional fields.
     */
    Join<Record, Agency> recordAgencyJoin = recordRoot.join(RecordTable.FIELD_AGENCY);
    //Some other joins

    //This is where I had the problem. 
    applyOrderBy(criteriaQuery, criteriaBuilder, recordRoot);

    /*
     * Specify which columns to select and their order.
     * criteriaQuery.multiselect(....);
     */              
    applyMultiSelect(recordRoot, recordAgencyJoin, /*other joins*/ criteriaQuery);

    /*
     * criteriaQuery.where(somePredicate);
     */
    applyFilter(recordFilter, criteriaQuery, criteriaBuilder,
            recordRoot, recordAgencyJoin /*, other joins*/);
    TypedQuery<Record> query = entityManager.<Record>createQuery(criteriaQuery);
    RepositoryUtils.applyPagination(query, page, pageSize);
    return query.getResultList();
}


private void applyOrderBy(CriteriaBuilder criteriaBuilder, Root<Record> recordRoot, CriteriaQuery<Record> criteriaQuery) {
    //Other fields to be added to the final sort.

    Order dateTimeDescOrder = criteriaBuilder.desc(recordRoot.get(RecordTable.FIELD_DATE_TIME));
    criteriaQuery.orderBy(dateTimeDescOrder /*, other orders by*/);
}

事实证明，首先显示带有 NULL dateTimeField 的记录。 我使用 Postrgres 数据库。 我会回答这个问题，因为我找到了解决方案。 这是一个类似的帖子。JPA Criteria 查询 API 和按 null 最后排序 https://stackoverflow.com/questions/31113030/jpa-criteria-query-api-and-order-by-null-last

我在这里给出了这个任务的答案。

首先，Postgres 默认情况下首先返回 null。

SELECT * FROM record ORDER BY date_time_field DESC;

https://stackoverflow.com/a/7621232/4587961 https://stackoverflow.com/a/7621232/4587961

SELECT * FROM record ORDER BY date_time_field DESC NULLS LAST;

第二，我必须改变申请订单依据 method

private void applyOrderBy(CriteriaBuilder criteriaBuilder, Root<Record> recordRoot, CriteriaQuery<Record> criteriaQuery) {
    //In the class code
    //private static final Date MIN_DATE = new Date(0L);
    final Date MIN_DATE = new Date(0L);

    //We treat records will NULL dateTimeField as if it was MIN_DATE.
    Order dateTimeDescOrder = criteriaBuilder.desc(
            //NULL values - last - WORKAROUND.
            criteriaBuilder.coalesce(recordRoot.get(RecordTable.FIELD_DATE_TIME), MIN_DATE));
    criteriaQuery.orderBy(dateTimeDescOrder);
}

Note, 标准生成器从 hibernate-jpa-2.1 开始。

/**
 * Create an expression that returns null if all its arguments
 * evaluate to null, and the value of the first non-null argument
 * otherwise.
 *
 * @param x expression
 * @param y value
 *
 * @return coalesce expression
 */
<Y> Expression<Y> coalesce(Expression<? extends Y> x, Y y);