JPA Criteria API group_concat用法

Question

我目前正在编写一个报告,其中一个字段需要group_concat.

CriteriaQuery<GameDetailsDto> criteriaQuery = criteriaBuilder
                .createQuery(GameDetailsDto.class);
Root<BetDetails> betDetails = criteriaQuery.from(BetDetails.class);
Expression<String> betSelection = betDetails.get("winningOutcome");
criteriaQuery.multiselect(
    // other fields to select
    criteriaBuilder.function("group_concat", String.class, betSelection),
    // other fields to select
    );
//predicate, where clause and other filters

TypedQuery<GameDetailsDto> typedQuery = entityManager.createQuery(criteriaQuery);

这会在行上抛出一个空指针异常:

TypedQuery<GameDetailsDto> typedQuery = entityManager.createQuery(criteriaQuery);

我错误地使用了criteriaBuilder的函数方法吗？
文件说:

function(String name, Class<T> type, Expression<?>... args);

Answer 1

我想通过Hibernate-jpa-mysql如何做到这一点:

1.)创建了一个扩展org.hibernate.dialect.function.SQLFunction的GroupConcatFunction类(这个用于单列group_concat)

public class GroupConcatFunction implements SQLFunction {

@Override
public boolean hasArguments() {
    return true;
}

@Override
public boolean hasParenthesesIfNoArguments() {
    return true;
}

@Override
public Type getReturnType(Type firstArgumentType, Mapping mapping)
        throws QueryException {
    return StandardBasicTypes.STRING;
}

@Override
public String render(Type firstArgumentType, List arguments,
        SessionFactoryImplementor factory) throws QueryException {
    if (arguments.size() != 1) {
        throw new QueryException(new IllegalArgumentException(
                "group_concat shoudl have one arg"));
    }
    return "group_concat(" + arguments.get(0) + ")";
}

}

2.)我创建延伸org.hibernate.dialect.MySQL5Dialect的CustomMySql5Dialect类和登记在步骤1中创建的GROUP_CONCAT类
3)在该应用的上下文,我更新的jpaVendorAdapter使用CustomMySql5Dialect作为databasePlatform
4)最后使用它

criteriaBuilder.function("group_concat", String.class,
        sampleRoot.get("sampleColumnName"))