如何使用postgres条板箱处理查询返回的可选值？ [英] How to handle an optional value returned by a query using the postgres crate?

问题描述

我正在尝试获取查询的值，但是该值可以为NULL，并且我不知道如何在Rust中处理它。这是我的代码：

I'm trying to get a value for a query, but this value can be NULL and I don't know how to handle it in Rust. Here is my code:

let stmt = conn.prepare("SELECT * FROM pictures").unwrap();

for row in stmt.query(&[]).unwrap() {
    let id: i32 = row.get("id");
    let author: String = row.get("author");
    let description: String = row.get("description");

    let rating: String = row.get("rating");

    let gps_lat: String = row.get("gps_lat");
    let gps_long: String = row.get("gps_long");
    let date_taken: chrono::NaiveDate = row.get("date_taken");

    println!("id        -> {}\n
           author      -> {}\n
           description -> {}\n
           rating      -> {}\n
           gps_lat     -> {}\n
           gps_long    -> {}\n
           date        -> {}\n
       ", id, author, description, rating, gps_lat, gps_long, date_taken);
}

当我执行代码时，第一张图片很好，因为评价列不't NULL。但是第二张图片失败了，并给了我 Conversion（WasNull），因为没有评级，我尝试将NULL转换为chrono :: NaiveDate。

When I execute the code, the first picture comes well because the rating column isn't NULL. But the second picture fails and gives me "Conversion(WasNull)", because there is not rating and I try to convert a NULL into a chrono::NaiveDate.

推荐答案

如中所述文档：

可空性

除了类型上面列出的 FromSql 是为
Option< T> 实现的，其中 T 实现 FromSql 。 Option< T> 表示
可为空的Postgres值。

Nullability

In addition to the types listed above, FromSql is implemented for Option<T> where T implements FromSql. An Option<T> represents a nullable Postgres value.

为可以为NULL的字段请求 Option< Type> ；那么该库将自动将NULL转换为 None ：

Request an Option<Type> for the field that can be NULL; then the library will automatically convert NULL to None:

let rating: Option<String> = row.get("rating");

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