java - JPA 中的 PostgreSQL 函数 string_agg

Question

在 PostgreSQL 中，string_agg(column, separator) 允许聚合一些字符串。我尝试将它与 JPA 一起使用，但它不是 JPA 标准函数。

注意:这不等同于CriteriaBuilder#concat()。

所以，我试图告诉 JPA 这个函数存在，就像这样:

public class StringAgg extends ParameterizedFunctionExpression<String> implements Serializable {

  public static final String NAME = "string_agg";

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

  @Override
  protected boolean isStandardJpaFunction() {
    return false;
  }

  public StringAgg(CriteriaBuilderImpl criteriaBuilder, Expression<String> expression, String separator) {
    super(criteriaBuilder, String.class, NAME, expression, new LiteralExpression(criteriaBuilder, separator));
  }
}

然后:

Expression<String> exprStr = ...
CriteriaBuilder cb = ...
cb.construct(MyClass.class, 
             myClass.get(MyClass_.name),
             myClass.get(MyClass_.surname),
             new StringAgg(cb, exprStr, "/"));

问题，我得到一个 NullPointerException!

java.lang.NullPointerException: null
at org.hibernate.internal.util.ReflectHelper.getConstructor(ReflectHelper.java:355) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.tree.ConstructorNode.resolveConstructor(ConstructorNode.java:179) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.tree.ConstructorNode.prepare(ConstructorNode.java:152) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.HqlSqlWalker.processConstructor(HqlSqlWalker.java:1028) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExpr(HqlSqlBaseWalker.java:2279) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExprList(HqlSqlBaseWalker.java:2145) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectClause(HqlSqlBaseWalker.java:1451) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:571) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:299) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:247) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:261) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:189) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:141) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:119) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:87) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:190) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.internal.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:288) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]
at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:223) ~[hibernate-core-4.3.0.Beta3.jar:4.3.0.Beta3]

调试器显示 cb.construct() 的最后一个Selection (new StringAgg(cb, exprStr, "/"))被忽略。因此，搜索到的构造函数是 MyClass(String,String) 而不是 MyClass(String, String, String)。

是不是StringAgg的实现有问题？有人已经尝试在 JPA 中使用 string_agg 了吗？

解决方案(感谢 vzamanillo)

扩展方言:

public class PGDialect extends PostgreSQLDialect{

  public PGDialect() {
    super();
    registerFunction("string_agg", new SQLFunctionTemplate( StandardBasicTypes.STRING, "string_agg(?1, ?2)"));
  }
}

在persistence.xml中使用它

<properties>
  <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>
  <property name="hibernate.dialect" value="path.to.PGDialect"/>

然后使用CriteriaBuilder#function():

Expression<String> exprStr = ...
CriteriaBuilder cb = ...
cb.construct(MyClass.class, 
             myClass.get(MyClass_.name),
             myClass.get(MyClass_.surname),
             cb.function( "string_agg", myColPath, cb.literal("delimiter" )));

为了简化它，我创建了一个辅助方法:

public static Expression<String> strAgg(CriteriaBuilder cb, Expression<String> expression, String delimiter) {
  return cb.function( "string_agg", String.class, expression, cb.literal(delimiter));
}

所以代码变成:

Expression<String> exprStr = ...
CriteriaBuilder cb = ...
cb.construct(MyClass.class, 
             myClass.get(MyClass_.name),
             myClass.get(MyClass_.surname),
             strAgg(cb, myColPath, "delimiter"));

Answer 1

也许这对你有帮助，

您可以在 JPA 条件查询中调用数据库函数。

CriteriaBuilder 接口(interface)有一个“function”方法。

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

Create an expression for the execution of a database function.

Parameters:
    name - function name
    type - expected result type
    args - function arguments
Returns:
    expression

然后您可以尝试创建一个 CriteriaBuilder 帮助器类来获取您可以在我们的条件查询中照常使用的普通条件表达式

public abstract class CriteriaBuilderHelper {

    private static final String PG_STRING_AGG  = "string_agg";

    /**
    * @param cb the CriteriaBuilder to use
    * @param toJoin the string to join
    * @param delimiter the string to use
    * @return Expression<String>
    */
    public static Expression functionStringAgg(CriteriaBuilder cb, String toJoin, String delimiter) {
        return cb.function(PG_STRING_AGG, 
            String.class,
            cb.literal(toJoin),
            cb.literal(delimiter))
        );
    }
}

或者你可以使用自定义方言来注册一个新函数

public class PGDialect extends PostgreSQLDialect{

    public PGDialect() {
        super();
        registerFunction("string_agg", new SQLFunctionTemplate( StandardBasicTypes.STRING, "string_agg(?1, ?2)"));
    }
}

并将其作为普通函数在您的 CriteriaBuilder 中使用

Expression<String> functionStringAgg = cb.function( "string_agg", String.class, 
                                cb.parameter(String.class, "toJoin" ), 
                                cb.parameter(String.class, "delimiter"));

毕竟不要忘记将参数值设置为您的 CriteriaQuery

setParameter( "toJoin", toJoin);
setParameter( "delimiter", delimiter);