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.

What should happen when there are duplicate values for name? Should it just generate invalid data? — Shepmaster
– Shepmaster, Commented Nov 2, 2018 at 17:29
Good question. I believe YAML suggests duplicate items get overwritten in order, thus the last occurrence is used, other occurrences are dropped. For my use case, this is perfect. — Tim Visée
– Tim Visée, Commented Nov 2, 2018 at 17:32
I'd probably just deserialize to a Map<String, PartialItem> and then transform it to a Vec<Item>; I assume you are looking to avoid the transient Map being created? — Shepmaster
– Shepmaster, Commented Nov 2, 2018 at 17:38
Correct. And I would also prefer not to implement an additional PartialItem struct. Using an Option<String> as name would be possible for this, although I don't think this is optimal. Maybe the transient Map is what I must go for if no better option is available. — Tim Visée
– Tim Visée, Commented Nov 2, 2018 at 18:06
You'll probably have to implement Deserialize it yourself serde.rs/deserialize-map.html — PitaJ
– PitaJ, Commented Nov 2, 2018 at 18:24

Stargateur · Accepted Answer · 2018-11-03 11:20:18Z

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.

MemAllox · Accepted Answer · 2025-01-03 16:57:43Z

Use serde_with and its #[serde(rename = "$key$")].

use serde_with::{serde_as, KeyValueMap}; #[derive(Serialize, Deserialize)] struct Item { #[serde(rename = "$key$")] name: String, some_field: usize, } // This would be the enclosing type #[serde_as] #[derive(Serialize, Deserialize)] struct MyMap( #[serde_as(as = "KeyValueMap<_>")] Vec<Item>, );

attdona · Accepted Answer · 2018-11-02 20:06:14Z

Define a default value for Item::name field

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

With this trick Itemcan be used both for deserializing and for transforming to a Vec of 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();

OP states that they want to "avoid the transient Map being created"

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