2 files changed, 125 insertions, 1 deletions

diff --git a/src/axum.rs b/src/axum.rs
index 0493b72..e9a3c17 100644
--- a/src/axum.rs
+++ b/src/axum.rs
@@ -96,6 +96,90 @@ where
     }
 }
 
+/// Extractor that differentiates between the absence and presence of the query string
+/// using `Option<T>`. Absence of query string encoded as `None`. Otherwise, it behaves
+/// identical to the `QsQuery`.
+///
+/// ## Example
+///
+/// ```rust
+/// # extern crate axum_framework as axum;
+/// use serde_qs::axum::OptionalQsQuery;
+/// use serde_qs::Config;
+/// use axum::{response::IntoResponse, routing::get, Router, body::Body};
+///
+/// #[derive(serde::Deserialize)]
+/// pub struct UsersFilter {
+///    id: Vec<u64>,
+/// }
+///
+/// async fn filter_users(
+///     OptionalQsQuery(info): OptionalQsQuery<UsersFilter>
+/// ) -> impl IntoResponse {
+///     match info {
+///         Some(info) => todo!("Select users based on query string"),
+///         None => { todo!("No query string provided")}
+///     }
+/// }
+///
+/// fn main() {
+///     let app = Router::<()>::new()
+///         .route("/users", get(filter_users));
+/// }
+#[derive(Clone, Copy, Default)]
+pub struct OptionalQsQuery<T>(pub Option<T>);
+
+impl<T> std::ops::Deref for OptionalQsQuery<T> {
+    type Target = Option<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T> std::ops::DerefMut for OptionalQsQuery<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<T: std::fmt::Debug> std::fmt::Debug for OptionalQsQuery<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+#[axum::async_trait]
+impl<T, S> FromRequestParts<S> for OptionalQsQuery<T>
+where
+    T: serde::de::DeserializeOwned,
+    S: Send + Sync,
+{
+    type Rejection = QsQueryRejection;
+
+    async fn from_request_parts(
+        parts: &mut axum::http::request::Parts,
+        state: &S,
+    ) -> Result<Self, Self::Rejection> {
+        let Extension(qs_config) = Extension::<QsQueryConfig>::from_request_parts(parts, state)
+            .await
+            .unwrap_or_else(|_| Extension(QsQueryConfig::default()));
+        if let Some(query) = parts.uri.query() {
+            let error_handler = qs_config.error_handler.clone();
+            let config: QsConfig = qs_config.into();
+            config
+                .deserialize_str::<T>(query)
+                .map(|query| OptionalQsQuery(Some(query)))
+                .map_err(|err| match error_handler {
+                    Some(handler) => handler(err),
+                    None => QsQueryRejection::new(err, StatusCode::BAD_REQUEST),
+                })
+        } else {
+            Ok(OptionalQsQuery(None))
+        }
+    }
+}
+
 #[derive(Debug)]
 /// Rejection type for extractors that deserialize query strings
 pub struct QsQueryRejection {
diff --git a/tests/test_axum.rs b/tests/test_axum.rs
index 16449b9..dd86a9e 100644
--- a/tests/test_axum.rs
+++ b/tests/test_axum.rs
@@ -8,7 +8,7 @@ extern crate axum_framework as axum;
 extern crate serde_qs as qs;
 
 use axum::{extract::FromRequestParts, http::StatusCode, response::IntoResponse};
-use qs::axum::{QsQuery, QsQueryConfig, QsQueryRejection};
+use qs::axum::{OptionalQsQuery, QsQuery, QsQueryConfig, QsQueryRejection};
 use serde::de::Error;
 
 fn from_str<'de, D, S>(deserializer: D) -> Result<S, D::Error>
@@ -132,3 +132,43 @@ fn test_custom_qs_config() {
         assert!(s.common.remaining);
     })
 }
+
+#[test]
+fn test_optional_query_none() {
+    futures::executor::block_on(async {
+        let req = axum::http::Request::builder()
+            .uri("/test")
+            .body(())
+            .unwrap();
+        let (mut req_parts, _) = req.into_parts();
+
+        let OptionalQsQuery(s) = OptionalQsQuery::<Query>::from_request_parts(&mut req_parts, &())
+            .await
+            .unwrap();
+
+        assert!(s.is_none());
+    })
+}
+
+#[test]
+fn test_optional_query_some() {
+    futures::executor::block_on(async {
+        let req = axum::http::Request::builder()
+            .uri("/test?foo=1&bars%5B%5D=3&limit=100&offset=50&remaining=true")
+            .extension(QsQueryConfig::new(5, false))
+            .body(())
+            .unwrap();
+
+        let (mut req_parts, _) = req.into_parts();
+        let OptionalQsQuery(s) = OptionalQsQuery::<Query>::from_request_parts(&mut req_parts, &())
+            .await
+            .unwrap();
+
+        let query = s.unwrap();
+        assert_eq!(query.foo, 1);
+        assert_eq!(query.bars, vec![3]);
+        assert_eq!(query.common.limit, 100);
+        assert_eq!(query.common.offset, 50);
+        assert!(query.common.remaining);
+    })
+}