Allow duplicate query parameters

[LuckyTurtleDev]

Summary

How can I allow a query parameter to occur a second time? I am only interested at the first value, but the second should not result a deserialize error.

The following request fails:

http://localhost:8080/?theme=dark&id=1234&theme=dark
Failed to deserialize query string: duplicate field `theme`

#[derive(Deserialize)]
pub struct QueryData {
    theme: Theme,
    id: u64
}

I have also try to use Vec<Theme> but this also not work:

Failed to deserialize query string: invalid type: string "dark", expected a sequence

axum version

0.7.1

[davidpdrsn]

axum uses https://crates.io/crates/serde_urlencoded under the hood so there isn't anything we can do about that. Either open an issue with them or try https://crates.io/crates/serde_qs which also has axum support. Not sure if serde_qs supports it but worth a shot.

[anarchodin]

I don't think they'll take kindly to an issue, given prior responses. This means serde_urlencoded is intentionally non-conformant with the application/x-www-form-urlencoded spec. It seems like the axum_extra implementation should be the default one.

[jplatte]

You can use axum_extra::extract::Query (not the same as axum::extract::Query) if you change your QueryData definition to

#[derive(Deserialize)]
pub struct QueryData {
    #[serde(default)]
    theme: Vec<Theme>,
    id: u64
}

It's also possible to write a custom deserialization function that only extracts the first item from a sequence, but that's not as simple.

[LuckyTurtleDev]

Thanks axum_extra::extract::Query works for me:

use axum_extra::extract::Query;

#[derive(Deserialize)]
pub struct QueryData {
    #[serde(default, deserialize_with = "take_first")]
    theme: Theme,
    id: u64
}

pub fn take_first<'de, D, T>(deserializer: D) -> Result<T, D::Error>
where
	D: de::Deserializer<'de>,
	T: Deserialize<'de> + Default
{
	let vec: Vec<T> = Vec::deserialize(deserializer)?;
	Ok(vec.into_iter().next().unwrap_or_default())
}

or with Option:

use axum_extra::extract::Query;

#[derive(Deserialize)]
pub struct QueryData {
    #[serde(default, deserialize_with = "take_first")]
    theme: Option<Theme>,
    id: u64
}

pub fn take_first<'de, D, T>(deserializer: D) -> Result<Option<T>, D::Error>
where
	D: de::Deserializer<'de>,
	T: Deserialize<'de>
{
	let vec: Vec<T> = Vec::deserialize(deserializer)?;
	Ok(vec.into_iter().next())
}

[jplatte]

Note that serde(default) means that if the parameter is not specified, the Default impl for Theme will be used. I usually include this in examples of Vec usage with serde_html_form / axum_extra::extract::Query because you often want to allow zero occurrences to be supported as well. You can leave out serde(default) if the parameter must be specified at least once.

[LuckyTurtleDev]

Because you often want to allow zero occurrences to be supported as well.

If I do not use default in the first example, the vector is not empty instead I get an error that Theme is missing.

Failed to deserialize query string: missing field `theme`

[jplatte]

Yes, exactly. If you want to enforce that it's specified at least once, don't use serde(default).