有没有办法告诉Serde使用struct字段作为地图的键？

Question

我有一个项目映射，我想序列化为一个结构列表，每个结构都有一个对应键的字段。

想象一下，有一个这样的YAML文件：

name_a:
    some_field: 0
name_b:
    some_field: 0
name_c:
    some_field: 0

以及类似的结构：

struct Item {
    name: String,
    some_field: usize,
}

我想将命名项反序列化为Vec<Item>而不是Map<String, Item>。项目名称（name_a，...）放在对象的name字段中Item。

我尝试了以下操作：

extern crate serde_yaml;
use std::fs::read_to_string;

let contents = read_to_string("file.yml").unwrap();
let items: Vec<Item> = serde_yaml::from_str(&contents).unwrap();

但是，这不起作用并产生invalid type: map, expected a sequence错误。

我宁愿避免创建一个Map<String, PartialItem>转换为的瞬态，Vec也宁愿不实现其他PartialItem结构。可以使用Option<String>as name，尽管我认为这不是最佳选择。

Answer 1

一种方法是自己反序列化地图：

use std::fmt;

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

struct ItemMapVisitor {}

impl ItemMapVisitor {
    fn new() -> Self {
        Self {}
    }
}

#[derive(Debug, Deserialize)]
struct SomeField {
    some_field: u32,
}

#[derive(Debug)]
struct Item {
    name: String,
    some_field: u32,
}

#[derive(Debug)]
struct VecItem(Vec<Item>);

impl Item {
    fn new(name: String, some_field: u32) -> Self {
        Self { name, some_field }
    }
}

impl<'de> Visitor<'de> for ItemMapVisitor {
    type Value = VecItem;

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

    fn visit_map<M>(self, mut access: M) -> Result<Self::Value, M::Error>
    where
        M: MapAccess<'de>,
    {
        let mut items = Vec::with_capacity(access.size_hint().unwrap_or(0));
        while let Some((key, value)) = access.next_entry::<String, SomeField>()? {
            items.push(Item::new(key, value.some_field));
        }
        Ok(VecItem(items))
    }
}

impl<'de> Deserialize<'de> for VecItem {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_map(ItemMapVisitor::new())
    }
}

fn main() {
    let contents = r#"
name_a:
    some_field: 0
name_b:
    some_field: 1
name_c:
    some_field: 2
"#;

    let items: VecItem = serde_yaml::from_str(&contents).unwrap();
    println!("{:#?}", items);
}

输出：

use std::fmt;

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

struct ItemMapVisitor {}

impl ItemMapVisitor {
    fn new() -> Self {
        Self {}
    }
}

#[derive(Debug, Deserialize)]
struct SomeField {
    some_field: u32,
}

#[derive(Debug)]
struct Item {
    name: String,
    some_field: u32,
}

#[derive(Debug)]
struct VecItem(Vec<Item>);

impl Item {
    fn new(name: String, some_field: u32) -> Self {
        Self { name, some_field }
    }
}

impl<'de> Visitor<'de> for ItemMapVisitor {
    type Value = VecItem;

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

    fn visit_map<M>(self, mut access: M) -> Result<Self::Value, M::Error>
    where
        M: MapAccess<'de>,
    {
        let mut items = Vec::with_capacity(access.size_hint().unwrap_or(0));
        while let Some((key, value)) = access.next_entry::<String, SomeField>()? {
            items.push(Item::new(key, value.some_field));
        }
        Ok(VecItem(items))
    }
}

impl<'de> Deserialize<'de> for VecItem {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_map(ItemMapVisitor::new())
    }
}

fn main() {
    let contents = r#"
name_a:
    some_field: 0
name_b:
    some_field: 1
name_c:
    some_field: 2
"#;

    let items: VecItem = serde_yaml::from_str(&contents).unwrap();
    println!("{:#?}", items);
}

如果你不想要Somefield结构。你也可以使用这个：

#[derive(Debug, Deserialize)]
struct Item {
    #[serde(skip)]
    name: String,
    some_field: u32,
}

while let Some((key, value)) = access.next_entry::<String, Item>()? {
    items.push(Item::new(key, value.some_field));
}

但这可能会添加一些无用的副本。

Answer 2

Item::name定义字段的默认值

#[derive(Debug, Serialize, Deserialize)]
struct Item {
    #[serde(default)]
    name: String,
    some_field: usize,
}

通过这个技巧，Item可以用于反序列化和转换为 a Vecof Items：

let contents = read_to_string("file.yml").unwrap();

let items: HashMap<String, Item> = serde_yaml::from_str(&contents).unwrap();

let slist: Vec<Item> = items
    .into_iter()
    .map(|(k, v)| Item {
        name: k,
        some_field: v.some_field,
    })
    .collect();