postgresql JPA - 从计算列设置实体类属性？
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JPA - Setting entity class property from calculated column?
提问by growse
I'm just getting to grips with JPA in a simple Java web app running on Glassfish 3 (Persistence provider is EclipseLink). So far, I'm really liking it (bugs in netbeans/glassfish interaction aside) but there's a thing that I want to be able to do that I'm not sure how to do.
我刚刚在 Glassfish 3 上运行的简单 Java Web 应用程序中掌握 JPA（持久性提供程序是 EclipseLink）。到目前为止，我真的很喜欢它（除了 netbeans/glassfish 交互中的错误），但有一件事情我希望能够做到，但我不知道该怎么做。
I've got an entity class (Article) that's mapped to a database table (article). I'm trying to do a query on the database that returns a calculated column, but I can't figure out how to set up a property of the Article class so that the property gets filled by the column value when I call the query.
我有一个映射到数据库表（文章）的实体类（文章）。我正在尝试对返回计算列的数据库进行查询，但我无法弄清楚如何设置 Article 类的属性，以便在调用查询时由列值填充该属性。
If I do a regular "select id,title,body from article" query, I get a list of Article objects fine, with the id, title and body properties filled. This works fine.
如果我执行常规的“从文章中选择 id、标题、正文”查询，我会得到一个很好的文章对象列表，其中填充了 id、标题和正文属性。这工作正常。
However, if I do the below:
但是，如果我执行以下操作：
Query q = em.createNativeQuery("select id,title,shorttitle,datestamp,body,true as published, ts_headline(body,q,'ShortWord=0') as headline, type from articles,to_tsquery('english',?) as q where idxfti @@ q order by ts_rank(idxfti,q) desc",Article.class);
(this is a fulltext search using tsearch2 on Postgres - it's a db-specific function, so I'm using a NativeQuery)
（这是在 Postgres 上使用 tsearch2 的全文搜索 - 这是一个特定于数据库的函数，所以我使用的是 NativeQuery）
You can see I'm fetching a calculated column, called headline. How do I add a headline property to my Article class so that it gets populated by this query?
您可以看到我正在获取一个名为标题的计算列。如何将标题属性添加到我的文章类，以便此查询填充它？
So far, I've tried setting it to be @Transient, but that just ends up with it being null all the time.
到目前为止，我已经尝试将它设置为@Transient，但最终它始终为空。
回答by axtavt
There are probably no good ways to do it, only manually:
可能没有好的方法可以做到，只能手动：
Object[] r = (Object[]) em.createNativeQuery(
"select id,title,shorttitle,datestamp,body,true as published, ts_headline(body,q,'ShortWord=0') as headline, type from articles,to_tsquery('english',?) as q where idxfti @@ q order by ts_rank(idxfti,q) desc","ArticleWithHeadline")
.setParameter(...).getSingleResult();
Article a = (Article) r[0];
a.setHeadline((String) r[1]);
-
——
@Entity
@SqlResultSetMapping(
name = "ArticleWithHeadline",
entities = @EntityResult(entityClass = Article.class),
columns = @ColumnResult(name = "HEADLINE"))
public class Article {
@Transient
private String headline;
...
}
回答by Pascal Thivent
AFAIK, JPA doesn't offer standardized support for calculated attributes. With Hibernate, one would use a Formulabut EclipseLink doesn't have a direct equivalent. James Sutherland made some suggestions in Re: Virtual columns (@Formula of Hibernate)though:
AFAIK，JPA 不提供对计算属性的标准化支持。使用 Hibernate，可以使用公式，但 EclipseLink 没有直接等效项。James Sutherland 在Re: Virtual columns (@Formula of Hibernate) 中提出了一些建议：
There is no direct equivalent (please log an enhancement), but depending on what you want to do, there are ways to accomplish the same thing.
EclipseLink defines a TransformationMapping which can map a computed value from multiple field values, or access the database.
You can override the SQL for any CRUD operation for a class using its descriptor's DescriptorQueryManager.
You could define a VIEW on your database that performs the function and map your Entity to the view instead of the table.
You can also perform minor translations using Converters or property get/set methods.
没有直接的等价物（请记录增强功能），但是根据您想要做什么，有多种方法可以完成相同的事情。
EclipseLink 定义了一个 TransformationMapping，它可以从多个字段值映射一个计算值，或者访问数据库。
您可以使用类的描述符的 DescriptorQueryManager 覆盖类的任何 CRUD 操作的 SQL。
您可以在执行该功能的数据库上定义一个 VIEW，并将您的实体映射到视图而不是表。
您还可以使用转换器或属性获取/设置方法执行次要翻译。
Also have a look at the enhancement requestthat has a solution using a DescriptorEventListenerin the comments.
还可以查看在评论中使用 a 的解决方案的增强请求DescriptorEventListener。
All this is non standard JPA of course.
当然，所有这些都是非标准的 JPA。
