如何使用条件查询仅选择外键值?

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假设我有两个实体:

@Entity public class A { @Id private int id; @ManyToOne private B b; //more attributes } @Entity public class B { @Id private int id; }

因此，A的表具有作为外键的列作为b_id.

So, the table for A is having a column as b_id as the foreign key.

现在，我想根据其他字段上的某些条件仅选择b_id.如何使用条件查询来做到这一点?

Now, I want to select just the b_id based on some criteria on other fields. How can I do this using criteria query?

我尝试执行以下操作，该操作引发IllegalArgumentException说"Unable to locate Attribute with the given name [b_id] on this ManagedType [A]"

I tried doing following which throws IllegalArgumentException saying "Unable to locate Attribute with the given name [b_id] on this ManagedType [A]"

CriteriaQuery<Integer> criteriaQuery = criteriaBuilder.createQuery(Integer.class); Root<A> root = criteriaQuery.from(A.class); Path<Integer> bId = root.get("b_id"); //building the criteria criteriaQuery.select(bId);

推荐答案

您需要加入B，然后获取id:

You need to join to B and then fetch the id:

Path<Integer> bId = root.join("b").get("id");