JPA &Criteria API - 仅选择特定列 [英] JPA & Criteria API - Select only specific columns
问题描述
我只想选择特定的列(例如 SELECT a FROM b).我有一个通用的 DAO，我想出的是:
I would like to select only specific columns (ex. SELECT a FROM b). I have a generic DAO and what I came up with is:
public List<T> getAll(boolean idAndVersionOnly) {
CriteriaBuilder builder = manager.getCriteriaBuilder();
CriteriaQuery<T> criteria = builder.createQuery(entityClazz);
Root<T> root = criteria.from(entityClazz);
if (idAndVersionOnly) {
criteria.select(root.get("ID").get("VERSION")); // HERE IS ERROR
} else {
criteria.select(root);
}
return manager.createQuery(criteria).getResultList();
}
错误是:CriteriaQuery.我应该如何改变它?我想获得一个类型 T 对象，它只有 ID 和 VERSION 字段，所有其他字段都是 null.
And the error is:
The method select(Selection<? extends T>) in the type CriteriaQuery<T> is not applicable for the arguments (Path<Object>). How should I change that? I want to get a type T object that has only ID and VERSION fields, and all others are null.
Type T 扩展了 AbstractEntity ，它有这两个字段.
Type T extends AbstractEntity which has those 2 fields.
entityClazz 是 T.class.
推荐答案
获取特定列的 JPA 方法之一是请求 元组对象.
One of the JPA ways for getting only particular columns is to ask for a Tuple object.
在您的情况下，您需要编写如下内容:
In your case you would need to write something like this:
CriteriaQuery<Tuple> cq = builder.createTupleQuery();
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION)); //using metamodel
List<Tuple> tupleResult = em.createQuery(cq).getResultList();
for (Tuple t : tupleResult) {
Long id = (Long) t.get(0);
Long version = (Long) t.get(1);
}
如果您有一个表示结果的类，则可以使用另一种方法，例如您的情况中的 T.T 不需要是实体类.如果 T 有一个构造函数，如:
Another approach is possible if you have a class representing the result, like T in your case. T doesn't need to be an Entity class. If T has a constructor like:
public T(Long id, Long version)
然后你可以直接在你的 CriteriaQuery 构造函数中使用 T :
then you can use T directly in your CriteriaQuery constructor:
CriteriaQuery<T> cq = builder.createQuery(T.class);
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION)); //using metamodel
List<T> result = em.createQuery(cq).getResultList();
请参阅此链接以获取进一步参考.
See this link for further reference.
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