如果字段为空，则 Jooq fetchInto 使用默认值 [英] Jooq fetchInto with default value if field is null

问题描述

我有数据类

data class Author(
    val id: String,
    val name: String,
    val books: MutableList<Book> = mutableListOf()
) {}

我用 jooq 写了请求

And I wrote request using jooq

val resultSet = dsl.select(author.ID, author.NAME,
            field(select(jsonArrayAgg(jsonObject(book.ID, book.PRICE)))
                .from(books)
                .where(book.AUTHOR_ID.eq(author.ID))
            ).`as`("books"))
            .from(authors)
            .fetchInto(Author::class.java)

但是如果作者没有书，我会得到一个例外.这是合乎逻辑的，因为数据类 Author 中的字段书籍不可为空.我试图将伴随对象添加到数据类

But I get an exception if Author has no books. It is logical, because field books in data class Author is not nullable. I tried to add companion object to data class

companion object {
        operator fun invoke(
            id: String,
            name: String,
            books: MutableList<Book>? = null
        ) = Author(id, name, books ?: mutableListOf())
    }

但是它不起作用，jooq 仍然尝试使用默认构造函数并且我得到一个异常.有没有办法在不使数据类 Author 中的外地手册可以为空的情况下修复它?

But it doesn't work, jooq still tries to use default constructor and I get an exception. Is there a way to fix it without making field books in data class Author nullable?

推荐答案

原因是 JSON_ARRAYAGG()(像大多数聚合函数一样)为空集产生 NULL，而不是更合理"的空 [].显然，你永远不想要这种行为.所以，你可以使用COALESCE，而另见这个问题:

The reason is that JSON_ARRAYAGG() (like most aggregate functions) produces NULL for empty sets, instead of a more "reasonable" empty []. Clearly, you never want this behaviour. So, you could use COALESCE, instead, see also this question:

coalesce(
  jsonArrayAgg(jsonObject(book.ID, book.PRICE)),
  jsonArray()
)

我会确保更新我在 Stack Overflow 上给出的所有其他答案以指出这一点.未来的 jOOQ 版本可能会提供 NULL 安全的聚合函数，以防存在聚合的合理标识(例如 [])以使其更容易被发现:https://github.com/jOOQ/jOOQ/issues/11994

I'll make sure to update all the other answers I've given on Stack Overflow to point this out. A future jOOQ version might offer NULL safe aggregate functions in case there exists a reasonable identity for aggregation (e.g. []) to make this more discoverable: https://github.com/jOOQ/jOOQ/issues/11994

这篇关于如果字段为空，则 Jooq fetchInto 使用默认值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！