java JPA Criteria Builder 中的 If/Case 语句
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If/Case statement in JPA Criteria Builder
提问by RNJ
I want to build up a query which search dates on different entites. My structure is:
我想建立一个查询，在不同的实体上搜索日期。我的结构是：
- Contract has date (non nullable)
- Employee has date (non nullable)
- Employee may have a contract id (nullable)
- 合同有日期（不可为空）
- 员工有日期（不可为空）
- 员工可能有一个合同 ID（可为空）
If a Employee has a contract I want to retrieve the contract date. If an employee does not have a contract then I want to return the Employee date.
如果员工有合同，我想检索合同日期。如果员工没有合同，那么我想返回员工日期。
My code so far is:
到目前为止我的代码是：
if (inputDate!= null) {
ParameterExpression<Date> exp = criteriaBuilder.parameter(Date.class, "inputDate");
criteria.add(criteriaBuilder.or(
criteriaBuilder.isNull(employee.get("contract")),
criteriaBuilder.lessThanOrEqualTo(employee.<Date>get("creationDate"), exp), criteriaBuilder.lessThanOrEqualTo((employee.join("contract").<Date>get("fromDate")), exp) ));}
This does not seem to work though. I always appear to go into the isNull which I do not expect.
但这似乎不起作用。我似乎总是进入我不期望的 isNull 。
I am happy to look into this some more but I guess my question is whether this is the correct way of going about it. Is it? I have seen a selectCase as well in criteriaBuilder so perhaps that may be a better solution.
我很高兴再研究一下，但我想我的问题是这是否是正确的做法。是吗？我在criteriaBuilder 中也看到了一个selectCase，所以这可能是一个更好的解决方案。
Any pointers would be greatly received.
任何指针都会受到极大的欢迎。
Thanks
谢谢
回答by JMelnik
Here would be a solution, not sure if it works, but we will manage to get it work with your help :):
这是一个解决方案，不确定它是否有效，但我们会在您的帮助下设法让它工作:)：
ParameterExpression<Date> inputDateExp = criteriaBuilder.parameter(Date.class, "inputDate");
Predicate employeeCreationDate = criteriaBuilder.lessThanOrEqualTo(
employee.<Date>get("creationDate"), inputDateExp);
Predicate contractStartDate = criteriaBuilder.lessThanOrEqualTo(
employee.join("contract").<Date>get("fromDate"), inputDateExp);
criteria.add(
criteriaBuilder.selectCase().when(employee.get("contract").isNull(), employeeCreationDate).otherwise(contractStartDate));
I do not understand why use "inputDate" as Expression instead of a Date? Also, I suggest renaming criteriato cand criteriaBuilderto cb, this would save space and I think it is more comfortable.
我不明白为什么使用“inputDate”作为表达式而不是日期？另外，我建议重命名criteria，以c和criteriaBuilder到cb，这样可节省空间，我认为这是更舒适。
回答by Patrick Garner
I think what you want to do is use Case<Date>, and use Predicateonly as first argument to Case#when. Case<Date>is an Expression<Date>, which is what you want to pass into CriteriaQuery#multiselect.
我认为您想要做的是使用Case<Date>，并且Predicate仅将其用作Case#when. Case<Date>是一个Expression<Date>，这是您想要传递的内容CriteriaQuery#multiselect。
CriteriaQuery<Employee> query = criteriaBuilder.createQuery(Employee.class);
Root<Employee> employee = query.from(Employee.class);
query.multiselect
(
criteriaBuilder.selectCase()
.when(criteriaBuilder.isNull(employee.get("contract")), employee.get("creationDate"))
.otherwise(employee.join("contract").get("fromDate"))
);
