JPA条件API最后按NULL顺序排序 [英] JPA criteria API order by NULL last

问题描述

我使用JPA条件API从日期库中获取记录。
我有实体记录，字段 dateTime ，可以为null。我会编码：

I use JPA criteria API to fetch records from the datebase. I have entity Record with field dateTime which can be null. I would code:

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。
这是一篇类似的帖子。
JPA Criteria Query API，最后按null排序

It turns out, records with NULL dateTimeField are shown first. I use Postrgres database. I will answer this question because I found a solution. Here is a similar post. JPA Criteria Query API and order by null last

推荐答案

这里我给出了这个任务的答案。

Here I put an answer to this task.

首先，Postgres默认首先返回空值。

First, Postgres by default returns nulls first.

SELECT * FROM record ORDER BY date_time_field DESC;

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

SELECT * FROM record ORDER BY date_time_field DESC NULLS LAST;

其次，我必须更改 applyOrderBy 方法

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);
}

注意，来自hibernate-jpa-2.1的 CriteriaBuilder 。

Note, CriteriaBuilder from hibernate-jpa-2.1.

/**
 * 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);

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