Question

我希望从Test::team_size对象本身的数据中反序列化Test属性：

#[derive(Debug, Serialize, Deserialize)]
struct TeamSize {
    pub min: i64,
    pub max: i64,
}

#[derive(Debug, Serialize, Deserialize)]
struct Test {
    pub i: i64,
    pub team_size: TeamSize,
}

fn main() {
    let t: Test = serde_json::from_str(r#"{"i": -2, "min": 2, "max": 5}"#).unwrap();
    assert_eq!(t.i, -2);
    assert_eq!(t.team_size.min, 2);
    assert_eq!(t.team_size.max, 5);
}

此代码无法编译，我不知道如何让Serde做我想做的事。有没有办法在这个例子中从原始结构的JSON反序列化team_size，它是一个子域？

似乎我想要喜欢 #[serde(untagged)]但是对于结构和字段而不是整个结构。

Answer 1

从Serde 1.0.34开始，您现在可以使用#[serde(flatten)]：

#[derive(Debug, Serialize, Deserialize)]
struct Test {
    pub i: i64,
    #[serde(flatten)]
    pub team_size: TeamSize,
}

在此之前，最简单的解决方法是将反序列化为私有帮助程序类型并根据需要对其进行重组：

#[macro_use]
extern crate serde_derive;

extern crate serde;
extern crate serde_json;

use serde::{Serialize, Serializer, Deserialize, Deserializer};

#[derive(Debug)]
pub struct TeamSize {
    pub min: i64,
    pub max: i64,
}

#[derive(Debug)]
pub struct Test {
    pub i: i64,
    pub team_size: TeamSize,
}

// Private helper
#[derive(Serialize, Deserialize)]
struct Helper {
    pub i: i64,
    pub min: i64,
    pub max: i64,
}

impl Serialize for Test {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where S: Serializer
    {
        let helper = Helper {
            i: self.i,
            min: self.team_size.min,
            max: self.team_size.max,
        };
        helper.serialize(serializer)
    }
}

impl<'de> Deserialize<'de> for Test {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer<'de>
    {
        let helper = Helper::deserialize(deserializer)?;
        Ok(Test {
            i: helper.i,
            team_size: TeamSize {
                min: helper.min,
                max: helper.max,
            },
        })
    }
}

fn main() {
    let j = r#" { "i": -2, "min": 2, "max": 5 } "#;

    let de: Test = serde_json::from_str(j).unwrap();
    println!("{:#?}", de);

    let ser = serde_json::to_string_pretty(&de).unwrap();
    println!("{}", ser);
}

Answer 2

Serde documentation有一章说明当自动生成的属性没有完全按照您的要求进行时implement custom deserialization。从那里开始，它不是太复杂，只是单调乏味：

extern crate serde;
extern crate serde_json;

use std::fmt;

use serde::de::{self, Deserialize, Deserializer, Visitor, MapAccess};

#[derive(Debug)]
struct TeamSize {
    pub min: i64,
    pub max: i64,
}

#[derive(Debug)]
struct Test {
    pub i: i64,
    pub team_size: TeamSize,
}

impl<'de> Deserialize<'de> for Test {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer<'de>
    {
        struct TestVisitor;

        impl<'de> Visitor<'de> for TestVisitor {
            type Value = Test;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("struct Test")
            }

            fn visit_map<V>(self, mut map: V) -> Result<Test, V::Error>
                where V: MapAccess<'de>
            {
                let mut min = None;
                let mut max = None;
                let mut i = None;

                while let Some(key) = map.next_key()? {
                    match key {
                        "min" => {
                            if min.is_some() {
                                return Err(de::Error::duplicate_field("min"));
                            }
                            min = Some(map.next_value()?);
                        }
                        "max" => {
                            if max.is_some() {
                                return Err(de::Error::duplicate_field("max"));
                            }
                            max = Some(map.next_value()?);
                        }
                        "i" => {
                            if i.is_some() {
                                return Err(de::Error::duplicate_field("i"));
                            }
                            i = Some(map.next_value()?);
                        }
                        _ => {
                            /* Ignore extra fields */
                        }
                    }
                }

                let min = min.ok_or_else(|| de::Error::missing_field("min"))?;
                let max = max.ok_or_else(|| de::Error::missing_field("max"))?;
                let i = i.ok_or_else(|| de::Error::missing_field("i"))?;

                Ok(Test { i, team_size: TeamSize { min, max }})
            }
        }

        const FIELDS: &'static [&'static str] = &["min", "max", "i"];
        deserializer.deserialize_struct("Test", FIELDS, TestVisitor)
    }
}

fn main() {
    let t: Test = serde_json::from_str(r#"{"i": -2, "min": 2, "max": 5}"#).unwrap();
    assert_eq!(t.i, -2);
    assert_eq!(t.team_size.min, 2);
    assert_eq!(t.team_size.max, 5);
}

如何使用Serde从原始结构的JSON反序列化结构的子字段？

2 个答案: