如何在JPA2中检查集合大小 [英] How to check a collection size in JPA2

问题描述

请考虑以下内容:

@Entity
public class Book 
{ 
    private List<String> authors;
    @ElementCollection
    public List<String> getAuthors() {
        return authors;
    }

    public void setAuthors(List<String> authors) {
        this.authors = authors;
    }
}

如何键入JPA2 CriteriaQuery表达式，例如，这将使我查找所有具有2位以上作者的所有Books?

How to type a JPA2 CriteriaQuery expression which, say, will let me find all the Books which have more than 2 authors?

推荐答案

在JPQL中:

select b from Book where size(b.authors) >= 2

使用标准API(但是为什么要用以下混乱内容替换这种简单的静态查询?):

Using the criteria API (but why would you replace such a simple static query with the following mess?):

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Book> criteriaQuery = cb.createQuery(Book.class);
Root<Book> book = criteriaQuery.from(Book.class);
Predicate predicate = cb.ge(cb.size(book.get(Book_.authors)), 2);
criteriaQuery.where(predicate);
criteriaQuery.select(book);

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