DateTime<Utc> 编译但不是 DateTime<Local> 查询表，其列定义为带时区的时间戳

Question

我有一个 postgresql 表，其列定义为 timestamp with time zone。表映射到这个结构：

#[derive(Serialize, Queryable)]
pub struct Location {
    pub publication_time: DateTime<Utc>,
    pub id: i32,
    pub name: String,
    pub latitude: BigDecimal,
    pub longitude: BigDecimal,
}

架构有这样的定义：

table! {
    locations {
        publication_time -> Timestamptz,
        id -> Integer,
        name -> Text,
        latitude -> Numeric,
        longitude -> Numeric,
    }
}

（部分）Cargo.toml：

serde = "1.0.125"
serde_json = "1.0.64"
serde_derive = "1.0.125"
diesel = { version = "1.4.6", features = ["postgres", "r2d2", "chrono", "numeric"] }
bigdecimal = { version = "0.1.0", features = ["serde"] }
chrono = { version = "0.4.19", features = ["serde"] }

查询数据库的函数：

fn get_all_locations(pool: web::Data<Pool>) -> Result<Vec<Location>, diesel::result::Error> {
    let conn = pool.get().unwrap();
    let items = locations.load::<Location>(&conn)?;
    Ok(items)
}

然后使用 serde_json 将其序列化为 JSON 数组。数据库中的日期时间是2021-04-08 15:02:02.514+02。当 DateTime 为 Utc 时，程序编译得很好，但以 UTC 显示的 DateTime 像 2021-04-08T13:02:02.514Z。我将 publication_time 更改为 DateTime<Local> 以保留时区信息，但随后 cargo build 失败：

error[E0277]: the trait bound `DateTime<Local>: FromSql<diesel::sql_types::Timestamptz, Pg>` is not satisfied
  --> src/controller.rs:21:27
   |
21 |     let items = locations.load::<Location>(&conn)?;
   |                           ^^^^ the trait `FromSql<diesel::sql_types::Timestamptz, Pg>` is not implemented for `DateTime<Local>`
   |
   = help: the following implementations were found:
             <DateTime<Utc> as FromSql<diesel::sql_types::Timestamptz, Pg>>
   = note: required because of the requirements on the impl of `diesel::Queryable<diesel::sql_types::Timestamptz, Pg>` for `DateTime<Local>`
   = note: 2 redundant requirements hidden
   = note: required because of the requirements on the impl of `diesel::Queryable<(diesel::sql_types::Timestamptz, diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Numeric, diesel::sql_types::Numeric), Pg>` for `models::Location`
   = note: required because of the requirements on the impl of `LoadQuery<_, models::Location>` for `locations::table`

我有另一个程序可以插入到这个表中，它可以工作，唯一的区别是派生（反序列化，可插入）。

#[derive(Deserialize, Insertable)]
pub struct Location {
    pub publication_time: DateTime<Local>,
    pub id: i32,
    pub name: String,
    pub latitude: BigDecimal,
    pub longitude: BigDecimal,
}

Answer 1

柴油机本身不支持将Timestamptz 字段映射到DateTime<Local>，因为它只为DateTime<Utc> 提供the corresponding impl。 您可以通过在相应字段上使用 #[diesel(deserialize_as = "…")] attribute 并提供您自己的反序列化包装器来解决此问题：

#[derive(Serialize, Queryable)]
pub struct Location {
    #[diesel(deserialize_as = "MyDateTimeWrapper")]
    pub publication_time: DateTime<Local>,
    pub id: i32,
    pub name: String,
    pub latitude: BigDecimal,
    pub longitude: BigDecimal,
}

pub struct MyDatetimeWrapper(DateTime<Local>);

impl Into<DateTime<Local>> for MyDatetimeWrapper {
    fn into(self) -> DateTime<Local> {
        self.0
    }
}

impl<DB, ST> Queryable<ST, DB> for MyDateTimeWrapper
where
    DB: Backend,
    DateTime<Utc>: Queryable<ST, DB>,
{
    type Row = <DateTime<Utc> as Queryable<ST, DB>>::Row;

    fn build(row: Self::Row) -> Self {
        Self(<DateTime<Utc> as Queryable<ST, DB>>::build(row).with_timezone(&Local))
    }
}