RUST-1945 Add a with_type method to the Aggregate action

src/action/aggregate.rs

@@ -1,4 +1,4 @@
-use std::time::Duration;
+use std::{marker::PhantomData, time::Duration};
 
 use bson::Document;
 
@@ -24,15 +24,17 @@ impl Database {
     /// See the documentation [here](https://www.mongodb.com/docs/manual/aggregation/) for more
     /// information on aggregations.
     ///
-    /// `await` will return d[`Result<Cursor<Document>>`] or d[`Result<SessionCursor<Document>>`] if
-    /// a `ClientSession` is provided.
+    /// `await` will return d[`Result<Cursor<Document>>`]. If a [`ClientSession`] was provided, the
+    /// returned cursor will be a [`SessionCursor`]. If [`with_type`](Aggregate::with_type) was
+    /// called, the returned cursor will be generic over the `T` specified.
     #[deeplink]
     pub fn aggregate(&self, pipeline: impl IntoIterator<Item = Document>) -> Aggregate {
         Aggregate {
             target: AggregateTargetRef::Database(self),
             pipeline: pipeline.into_iter().collect(),
             options: None,
             session: ImplicitSession,
+            _phantom: PhantomData,
         }
     }
 }
@@ -46,15 +48,17 @@ where
     /// See the documentation [here](https://www.mongodb.com/docs/manual/aggregation/) for more
     /// information on aggregations.
     ///
-    /// `await` will return d[`Result<Cursor<Document>>`] or d[`Result<SessionCursor<Document>>`] if
-    /// a [`ClientSession`] is provided.
+    /// `await` will return d[`Result<Cursor<Document>>`]. If a [`ClientSession`] was provided, the
+    /// returned cursor will be a [`SessionCursor`]. If [`with_type`](Aggregate::with_type) was
+    /// called, the returned cursor will be generic over the `T` specified.
     #[deeplink]
     pub fn aggregate(&self, pipeline: impl IntoIterator<Item = Document>) -> Aggregate {
         Aggregate {
             target: AggregateTargetRef::Collection(CollRef::new(self)),
             pipeline: pipeline.into_iter().collect(),
             options: None,
             session: ImplicitSession,
+            _phantom: PhantomData,
         }
     }
 }
@@ -66,8 +70,10 @@ impl crate::sync::Database {
     /// See the documentation [here](https://www.mongodb.com/docs/manual/aggregation/) for more
     /// information on aggregations.
     ///
-    /// [`run`](Aggregate::run) will return d[`Result<crate::sync::Cursor<Document>>`] or
-    /// d[`Result<crate::sync::SessionCursor<Document>>`] if a [`ClientSession`] is provided.
+    /// [`run`](Aggregate::run) will return d[Result<crate::sync::Cursor<Document>>`]. If a
+    /// [`crate::sync::ClientSession`] was provided, the returned cursor will be a
+    /// [`crate::sync::SessionCursor`]. If [`with_type`](Aggregate::with_type) was called, the
+    /// returned cursor will be generic over the `T` specified.
     #[deeplink]
     pub fn aggregate(&self, pipeline: impl IntoIterator<Item = Document>) -> Aggregate {
         self.async_database.aggregate(pipeline)
@@ -84,8 +90,10 @@ where
     /// See the documentation [here](https://www.mongodb.com/docs/manual/aggregation/) for more
     /// information on aggregations.
     ///
-    /// [`run`](Aggregate::run) will return d[`Result<crate::sync::Cursor<Document>>`] or
-    /// d[`Result<crate::sync::SessionCursor<Document>>`] if a `ClientSession` is provided.
+    /// [`run`](Aggregate::run) will return d[Result<crate::sync::Cursor<Document>>`]. If a
+    /// `crate::sync::ClientSession` was provided, the returned cursor will be a
+    /// `crate::sync::SessionCursor`. If [`with_type`](Aggregate::with_type) was called, the
+    /// returned cursor will be generic over the `T` specified.
     #[deeplink]
     pub fn aggregate(&self, pipeline: impl IntoIterator<Item = Document>) -> Aggregate {
         self.async_collection.aggregate(pipeline)
@@ -95,14 +103,15 @@ where
 /// Run an aggregation operation.  Construct with [`Database::aggregate`] or
 /// [`Collection::aggregate`].
 #[must_use]
-pub struct Aggregate<'a, Session = ImplicitSession> {
+pub struct Aggregate<'a, Session = ImplicitSession, T = Document> {
     target: AggregateTargetRef<'a>,
     pipeline: Vec<Document>,
     options: Option<AggregateOptions>,
     session: Session,
+    _phantom: PhantomData<T>,
 }
 
-impl<'a, Session> Aggregate<'a, Session> {
+impl<'a, Session, T> Aggregate<'a, Session, T> {
     option_setters!(options: AggregateOptions;
         allow_disk_use: bool,
         batch_size: u32,
@@ -130,15 +139,50 @@ impl<'a> Aggregate<'a, ImplicitSession> {
             pipeline: self.pipeline,
             options: self.options,
             session: ExplicitSession(value.into()),
+            _phantom: PhantomData,
         }
     }
 }
 
-#[action_impl(sync = crate::sync::Cursor<Document>)]
-impl<'a> Action for Aggregate<'a, ImplicitSession> {
+impl<'a, Session> Aggregate<'a, Session, Document> {
+    /// Use the provided type for the returned cursor.
+    ///
+    /// ```rust
+    /// # use futures_util::TryStreamExt;
+    /// # use mongodb::{bson::Document, error::Result, Cursor, Database};
+    /// # use serde::Deserialize;
+    /// # async fn run() -> Result<()> {
+    /// # let database: Database = todo!();
+    /// # let pipeline: Vec<Document> = todo!();
+    /// #[derive(Deserialize)]
+    /// struct PipelineOutput {
+    ///     len: usize,
+    /// }
+    ///
+    /// let aggregate_cursor = database
+    ///     .aggregate(pipeline)
+    ///     .with_type::<PipelineOutput>()
+    ///     .await?;
+    /// let aggregate_results: Vec<PipelineOutput> = aggregate_cursor.try_collect().await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn with_type<T>(self) -> Aggregate<'a, Session, T> {
+        Aggregate {
+            target: self.target,
+            pipeline: self.pipeline,
+            options: self.options,
+            session: self.session,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+#[action_impl(sync = crate::sync::Cursor<T>)]
+impl<'a, T> Action for Aggregate<'a, ImplicitSession, T> {
     type Future = AggregateFuture;
 
-    async fn execute(mut self) -> Result<Cursor<Document>> {
+    async fn execute(mut self) -> Result<Cursor<T>> {
         resolve_options!(
             self.target,
             self.options,

src/test/coll.rs

@@ -27,6 +27,7 @@ use crate::{
     results::DeleteResult,
     test::{get_client_options, log_uncaptured, util::TestClient, EventClient},
     Collection,
+    Cursor,
     IndexModel,
 };
 
@@ -1306,3 +1307,45 @@ async fn insert_many_document_sequences() {
     let second_batch_len = second_event.command.get_array("documents").unwrap().len();
     assert_eq!(first_batch_len + second_batch_len, total_docs);
 }
+
+#[tokio::test]
+async fn aggregate_with_generics() {
+    #[derive(Serialize)]
+    struct A {
+        str: String,
+    }
+
+    #[derive(Deserialize)]
+    struct B {
+        len: i32,
+    }
+
+    let client = TestClient::new().await;
+    let collection = client
+        .database("aggregate_with_generics")
+        .collection::<A>("aggregate_with_generics");
+
+    let a = A {
+        str: "hi".to_string(),
+    };
+    let len = a.str.len();
+    collection.insert_one(&a).await.unwrap();
+
+    // Assert at compile-time that the default cursor returned is a Cursor<Document>
+    let basic_pipeline = vec![doc! { "$match": { "a": 1 } }];
+    let _: Cursor<Document> = collection.aggregate(basic_pipeline).await.unwrap();
+
+    // Assert that data is properly deserialized when using with_type
+    let project_pipeline = vec![doc! { "$project": {
+            "str": 1,
+            "len": { "$strLenBytes": "$str" }
+        }
+    }];
+    let cursor = collection
+        .aggregate(project_pipeline)
+        .with_type::<B>()
+        .await
+        .unwrap();
+    let lens: Vec<B> = cursor.try_collect().await.unwrap();
+    assert_eq!(lens[0].len as usize, len);
+}

src/test/db.rs

@@ -1,6 +1,7 @@
 use std::cmp::Ord;
 
 use futures::stream::TryStreamExt;
+use serde::Deserialize;
 
 use crate::{
     action::Action,
@@ -17,6 +18,7 @@ use crate::{
     results::{CollectionSpecification, CollectionType},
     test::util::TestClient,
     Client,
+    Cursor,
     Database,
 };
 
@@ -378,3 +380,32 @@ async fn clustered_index_list_collections() {
         .unwrap();
     assert!(clustered_index_collection.options.clustered_index.is_some());
 }
+
+#[tokio::test]
+async fn aggregate_with_generics() {
+    #[derive(Deserialize)]
+    struct A {
+        str: String,
+    }
+
+    let client = TestClient::new().await;
+    let database = client.database("aggregate_with_generics");
+
+    if client.server_version_lt(5, 1) {
+        log_uncaptured(
+            "skipping aggregate_with_generics: $documents agg stage only available on 5.1+",
+        );
+        return;
+    }
+
+    // The cursor returned will contain these documents
+    let pipeline = vec![doc! { "$documents": [ { "str": "hi" } ] }];
+
+    // Assert at compile-time that the default cursor returned is a Cursor<Document>
+    let _: Cursor<Document> = database.aggregate(pipeline.clone()).await.unwrap();
+
+    // Assert that data is properly deserialized when using with_type
+    let mut cursor = database.aggregate(pipeline).with_type::<A>().await.unwrap();
+    assert!(cursor.advance().await.unwrap());
+    assert_eq!(&cursor.deserialize_current().unwrap().str, "hi");
+}