我需要使用大量条件联接和 where 子句进行条件查询,在这种情况下,代码往往会变得复杂并且可能会产生重复的联接。
例如,我有以下表结构和 JPA 实体:
ACCOUNT
ACCOUNT_ID
ACCOUNT_TYPE
PERSON
NAME
AGE
ACCOUNT_ID ( FK TO ACCOUNT )
ADDRESS_ID ( FK TO ADDRESS )
ADDRESS
ADDRESS_ID
LOCATION
COUNTRY
所以假设我使用静态元模型实现来应用标准查询。
这是可以生成重复连接的错误代码示例:
CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Account> cq = cb.createQuery(Account.class);
cq.select(accountRoot).where(
cb.and(
cb.equal(accountRoot.join(Account_.person).get(Person_.name),"Roger"),
cb.greaterThan(accountRoot.join(Account_.person).get(Person_.age),18),
cb.equal(accountRoot.join(Account_.person)
.join(Person_.address).get(Address_.country),"United States")
)
)
TypedQuery<Account> query = entityManager.createQuery(cq);
List<Account> result = query.getResultList();
上面的代码将生成一个 SQL 与同一个表的多个连接:
Select
account0_.account_id as account1_2_,
account0_.account_type as account2_2_
from
account account0_
inner join
person person1_
on account0_.account_id=person1_.account_id
inner join
address address2_
on person1_.address_id=address2_.address_id
inner join
person person3_
on account0_.account_id=person3_.account_id
inner join
person person4_
on account0_.account_id=person4_.account_id
inner join
person person5_
on account0_.account_id=person5_.account_id
inner join
address address6_
on person5_.address_id=address6_.address_id
where
person3_.name=?
and person4_.age>18
and address6_.country=?
一个简单的解决方案是保留 Joins 的实例以在多个谓词中重用,例如:
Root<Account> accountRoot = cq.from(Account.class);
Join<Account,Person> personJoin= accountRoot.join(Account_.person);
Join<Person,Address> personAddressJoin = accountRoot.join(Person_.address);
cq.select(accountRoot).where(
cb.and(
cb.equal(personJoin.get(Person_.name),"Roger"),
cb.greaterThan(personJoin.get(Person_.age),18),
cb.equal(personAddressJoin.get(Address_.country),"United States")
)
)
好的,它可以工作,但是对于具有多个表和代码条件连接的真正复杂代码往往会变成意大利面条代码!相信我 !
避免它的更好方法是什么?