比较JPA条件API中的日期实体 [英] Compare Date entities in JPA Criteria API

问题描述

使用JPA 2与EclipseLink实现。

我正在尝试构建一个动态查询，这应该会使我在一个给定日期。

I'm trying to build a dynamic query which should bring me some records persisted after a given date.

CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Event> criteria = builder.createQuery(Event.class);
Root<Event> root = criteria.from(Event.class);
criteria.select(root);
criteria.distinct(true);
List<Predicate> predicates = new ArrayList<Predicate>();
//...
if (dateLimit != null){
    ParameterExpression<Date> param = builder.parameter(Date.class, "dateLimit");
    predicates.add(builder.lessThanOrEqualTo(root.get("dateCreated"), param));
}

lessThanOrEqualTo（）和 le（）是API中唯一可以帮助我的两种方法。这个警告是由eclipse抛出的：

lessThanOrEqualTo() and le() are the only two methods in the API which look like may help me in this case. This warning is thrown by the eclipse though:

Bound mismatch: The generic method lessThanOrEqualTo(Expression<? extends Y>, Expression<? extends Y>)
of type CriteriaBuilder is not applicable for the arguments (Path<Object>, ParameterExpression<Date>).
The inferred type Object is not a valid substitute for the bounded parameter
<Y extends Comparable<? super Y>>

我可以想像，我没有采取正确的方法来解决这个问题，但我找不到任何一个可能的解决方案的提示或指针。

I can imagine that I'm not taking the correct approach for this problem but I can't find anywhere some tips or pointers for a possible solution.

推荐答案

问题是，使用基于字符串的API，它不能推断类型对于获取 -Operation的结果值。这在例如

The problem is that with the string-based API it cannot infer the type for the result value of the get-Operation. This is explained for example in Javadoc for Path.

如果您使用

predicates.add(builder.lessThanOrEqualTo(root.<Date>get("dateCreated"), param));

相反，它可以正常工作，因为它可以从类型参数中找出返回类型，发现它是可比的。注意，使用参数化方法调用 root。< Date> get（...）（参见，例如，什么时候参数化方法调用有用？）。

instead, it will work fine, because it can figure out the return type from the type argument and will find out that it is comparable. Note, the use of a parameterised method invocation root.<Date>get(...) (see, e.g., When is a parameterized method call useful?).

另一个（在我看来更好）的解决方案是使用基于元模型的API而不是基于字符串的API。给出了一个关于规范元模型的简单示例，例如这里。如果您有更多的时间投资，这是关于静态元模型的好文章：动态，JPA 2.0中的类型安全查询

Another (in my opinion better) solution is to use the metamodel based API instead of the string-based one. A simple example about canonical metamodel is given for example here. If you have more time to invest, this is a good article about static metamodel: Dynamic, typesafe queries in JPA 2.0

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