Is there a way to tell Serde to use a struct field as a map's key?

Question

I have a map of items that I would like to serialize to a list of structs, each having a field for the corresponding key.

Imagine having a YAML file like this:

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

And a corresponding structure like this:

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

I would like to deserialize the named items into a Vec<Item> instead of a Map<String, Item>. The item names (name_a, ...) are put into the name field of the Item objects.

I've attempted the following:

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();

This however doesn't work and produces the invalid type: map, expected a sequence error.

I'd prefer to avoid creating a transient Map<String, PartialItem> that is converted to a Vec, and I would also prefer not to implement an additional PartialItem struct. Using an Option<String> as name would be possible, although I don't think this is optimal.

Answer 1

One way is to deserialize the map yourself:

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);
}

Output:

VecItem(
    [
        Item {
            name: "name_a",
            some_field: 0
        },
        Item {
            name: "name_b",
            some_field: 1
        },
        Item {
            name: "name_c",
            some_field: 2
        }
    ]
)

---

If you don't want of Somefield structure. You could also use this:

#[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));
}

But this could add some useless copy.