Example of Vec<String> in Query?

[gd87429]

I looked at #434 but still have no idea how I'm supposed to make something like this work in 0.6.2:

#[derive(Deserialize)]
pub struct Q {
	a: Vec<String>,
}

pub async fn handler(
	Query(payload): Query<Q>,
) -> Result {
...

No matter if I try ?a=b&a=c or ?a[]=b&a[]=c or ?a=b,c none of it works; I keep getting:

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

Is there an example somewhere I'm missing?

[davidpdrsn]

There is no standard way to encode collections as query params, therefore serde_url_encoded doesn't support it. That is the parser axum's Query uses.

With that said you can use axum_extra::extract::Query instead which expects params the way HTML forms encodes them. Example here https://github.com/tokio-rs/axum/blob/main/axum-extra/src/extract/query.rs#L147. You can also use serde_qs which does support the a[] syntax.

[gd87429]

Ahh, that's what I was missing. I get it now. Thank you very much for explaining & taking the time!

[JuxhinDB]

While the answer works, I'm noticing some intermittent behaviour when an array of one item is passed. Take the following simplified example.

#[derive(Debug, Deserialize)]
pub(crate) Params {
     pub(crate): some_flag: Option<bool>,

     #[serde(flatten)]
     pub(crate): filters: Filters,
}

#[derive(Debug, Deserialize)]
pub(crate) Filters {
     #[serde(rename = "filter")]
     pub(crate) filters: Option<Vec<String>>,
}

Passing ?filter=foo&filter=bar correctly parses this into Some(["foo", "bar"]). However just passing ?filter=foo does not parse this into Some(["foo"]), instead it fails and returns None. Internal query rejection issues the same error:

Error { inner: Error("invalid type: string \"foo\", expected a sequence") })

Not sure if this is expected behaviour?

[jplatte]

Serde's data model isn't exactly a great fit for query parameters. I think in this specific case it would help to change

#[serde(rename = "filter")]
pub(crate) filters: Option<Vec<String>>,

to

#[serde(default, rename = "filter")]
pub(crate) filters: Vec<String>,

[JuxhinDB]

Thanks @jplatte! That seems to do the trick. Perhaps we can extend the answer to include this.

Edit: apologies, turns out it seemed like it was working due to the default. Still the same behaviour.

[JuxhinDB]

It might help to clarify that in my actual use-case the internal type if not Vec<String> but rather a custom type that impls Deserialize.

[jplatte]

Can you post a new issue on https://github.com/jplatte/serde_html_form please that fully reproduces the problem? If it's reproducible without flatten and with default, that would be useful.

[JuxhinDB]

I can confirm that this only happens when using flatten. I've opened the issue here jplatte/serde_html_form#6 and have a PoC repo available here - https://github.com/JuxhinDB/serde-html-form-bug/blob/main/src/main.rs