是否使用CriteriaBuilder在子查询中选择路径？

Question

SELECT e FROM Entity e 
WHERE e.type = 'foo' 
AND EXISTS (SELECT a FROM e.attributes a 
             WHERE a.key = 'bar' 
               AND a.value = 'baz')

EntityManager em = /* ... */;
TypedQuery<Entity> tq = em.createQuery(queryString, Entity.class);

在实际操作中，查询中存在多个查询，因此需要使用CriteriaBuilder构造查询。到目前为止，我得到的最接近的方法是使子查询从Attributes a WHERE...中选择a，但这不限于e.Attributes，当然：

CriteriaBuilder cb = em.getCriteriaBuilder();

CriteriaQuery<Entity> query = cb.createQuery(Entity.class);
Root<Entity> root = query.from(Entity.class);

Subquery<Attribute> subquery = query.subquery(Attribute.class);
Root<Attribute> subroot = subquery.from(Attribute.class); // too broad
subquery.select(subroot)
    .where(cb.and(//
        cb.equal(subroot.get("key"), cb.literal("bar")),
        cb.equal(subroot.get("value"), cb.literal("baz"))));

query.select(root)
    .where(cb.and(//
        cb.equal(root.get("type"), cb.literal("foo")), //
        cb.exists(subquery)));

在Subquery上有许多correlate()方法，我想知道是否需要将实体与其属性连接到外部查询中，然后以某种方式将correlate()，但我从EE7 javadocs中不确定correlate()到底做了什么（但correlate()返回一个from，这意味着我可以从中进行选择，这是很有希望的）。

Answer 1

我最终在JPA2.0规范中找到了这个问题的答案。第276页有一个相关查询的示例：

为了表示一些涉及单向关系的相关子查询，将子查询的域与包含查询的域相关起来可能是有用的。这是通过使用子查询接口的correlate方法执行的。

例如：

CriteriaQuery<Customer> q = cb.createQuery(Customer.class);
Root<Customer> customer = q.from(Customer.class);
Subquery<Long> sq = q.subquery(Long.class);
Root<Customer> customerSub = sq.correlate(customer);
Join<Customer,Order> order = customerSub.join(Customer_.orders);
q.where(cb.gt(sq.select(cb.count(order)), 10))
.select(customer);

SELECT c
FROM Customer c
WHERE (SELECT COUNT(o) FROM c.orders o) > 10

from_clause ::=  FROM  identification_variable_declaration  ...
identification_variable_declaration ::= range_variable_declaration ...
range_variable_declaration ::= entity_name [AS] identification_variable

subquery_from_clause ::= FROM subselect_identification_variable_declaration ...
subselect_identification_variable_declaration ::=
  identification_variable_declaration |
  derived_path_expression [AS] identification_variable {join}* |
  derived_collection_member_declaration
derived_path_expression ::=
  superquery_identification_variable.{single_valued_object_field.}*collection_valued_field |
  superquery_identification_variable.{single_valued_object_field.}*single_valued_object_field