将 SQL 查询实现为 JPA 两个多对一关系的标准

Question

I am trying to build JPA criteria for the following scenario. I have the following 3 Entity classes, two of which have a @ManyToOne relationship to UniqueType, but UniqueType does not have an explicit relationship to any other class: The structure is as follows:

-------------           -----------            ------------ 
   event         N:1    unique_type     1:N       detail
-------------           -----------            ------------
event_id (PK)           type_code (PK)         detail_id (PK)
type_code (FK)                                 type_code (FK)
                                               abbreviation

The SQL query that successfully selects the Event records filtered by the abbreviation:

SELECT
  event.event_id,
  event.type_code,
  detail.abbreviation
FROM
  event 
  JOIN unique_type ON event.type_code = unique_type.type_code
  JOIN detail ON detail.type_code = unique_type.type_code
WHERE detail.abbreviation = 'ABC'

I have the Event class as my root, but when I join to the UniqueType class, I am stuck because there is no explicit relationship to Detail on UniqueType

Join<Event, UniqueType> joinToUniqueType = root.join(Event_.typeCode);
Join<UniqueType, ???>

I want to be able to search on abbreviation something like:

predicates.add(builder.lower(joinToDetail.get(Detail_.abbreviation)),'ABC');

How can I write the criteria? Any help is much appreciated.

Answer 1

Since you want inner join semantics, you can use cross joins instead which the database will optimize away anyway. Use the following

Root<Detail> event = query.from(Event.class);
Root<Detail> detail = query.from(Detail.class);

predicates.add(builder.eq(detail.get(Detail_.typeCode), event.get(Event_.typeCode));
predicates.add(builder.lower(detail.get(Detail_.abbreviation)),'ABC');