[英]How to query an M:N relationship with JPA2?
I have an an object (BlogPost) that contains an M:N collection of elements (Tags). 我有一个对象(BlogPost),它包含一个M:N元素集合(标签)。
How to query for an object (BlogPost) where at least one it its Tags matches an element in a set of Tags (defined by the user) with JPA2 (Hibernate). 如何查询一个对象(BlogPost),其中至少有一个对象的标签与一组标签(由用户定义)中的元素与JPA2(Hibernate)匹配。
findBlogPostWithAtLeastOneMatchingTag(Collection<Tag> tags){ ???? }
My main problem is, that I actually need to compare two collections of tags: - the collection of tags of the BlogPost. 我的主要问题是,我实际上需要比较两个标签集合: - BlogPost的标签集合。 - the collection I search for
- 我搜索的集合
I tried Select p from Post p where p.tags in(:tags)
but it does not work, as my post entities have more than just one tag. 我尝试
Select p from Post p where p.tags in(:tags)
但它不起作用,因为我的帖子实体只有一个标签。
So what could I do instead? 那我该怎么做呢?
My BlogPost entity looks like this. 我的BlogPost实体看起来像这样。 It has several Tags.
它有几个标签。
@Entity
public class BlogPost{
/** The tags. */
@ManyToMany()
@NotNull
private Set<Tag> tags;
@NotBlank
private String content;
...
}
The solution must not be JPQL, JPA-Criteria (not Hibernate-Criteria) would be fine too. 解决方案不能是JPQL,JPA-Criteria(不是Hibernate-Criteria)也可以。
If you like JPA Criteria, this is the solution for you: 如果你喜欢JPA Criteria,这就是你的解决方案:
List<Integer> myTagsIds = new ArrayList<Integer> ();
myTagsIds.add(1);
myTagsIds.add(2);
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<BlogPost> cq = cb.createQuery(BlogPost.class);
Root<BlogPost> blogPost = cq.from(BlogPost.class);
SetJoin<BlogPost, Tag> tags = blogPost.join(BlogPost_.tags);
Predicate predicate = tags.get(Tag_.id).in(myTagsIds);
cq.distinct(true);
cq.where(predicate);
TypedQuery<BlogPost> tq = em.createQuery(cq);
return tq.getResultList();
This solution makes use of the canonical MetaModel classes BlogPost_
and Tag_
that should be generated by your JPA implementation. 此解决方案使用应由JPA实现生成的规范MetaModel类
BlogPost_
和Tag_
。
In SQL it could be something like: 在SQL中它可能是这样的:
SELECT p FROM Post p WHERE (p.tags INTERSECT :tags IS NOT EMPTY);
Then apply the @SqlResultSetMapping
. 然后应用
@SqlResultSetMapping
。
You can use Criteria API
and start as you did but make a loop over Collection<Tag> tags
as: 您可以使用
Criteria API
并按照您的方式启动,但是对Collection<Tag> tags
进行循环:
* make a union of single query results from `Select p from Post p where p.tags in(:tags)`;
* take distinct over result of union.
Query will be server-side and you wouldn't have to do dirty work in Java. 查询将是服务器端,您不必在Java中进行脏工作。
You could do something like 你可以做点什么
Select t from Post t where t.tag in (select p.tag from Post p where p.id=:id) 从Post t中选择t,其中t.tag in(从post p中选择p.tag,其中p.id =:id)
id is the id of the current post. id是当前帖子的ID。 Basically you are selecting post with tags that are in the tags of current post.
基本上,您选择的帖子中包含当前帖子标签中的标签。
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