feat(application): add get_thought_view + get_thread_views use cases with real engagement stats

crates/application/Cargo.toml

@@ -14,6 +14,7 @@ sha2        = "0.10"
 hex         = "0.4"
 tracing     = { workspace = true }
 url         = { workspace = true }
+tokio       = { workspace = true }
 
 [dev-dependencies]
 tokio  = { workspace = true, features = ["full"] }

crates/application/src/use_cases/auth.rs

@@ -104,7 +104,7 @@ mod tests {
     use domain::{
         errors::DomainError,
         events::DomainEvent,
-        models::{feed::UserSummary, user::User},
+        models::{feed::{PageParams, Paginated, UserSummary}, user::User},
         ports::{AuthService, GeneratedToken, PasswordHasher, UserReader, UserWriter},
         testing::{NoOpEventPublisher, TestStore},
         value_objects::{Email, PasswordHash, UserId, Username},
@@ -135,6 +135,12 @@ mod tests {
         async fn count(&self) -> Result<i64, DomainError> {
             self.0.count().await
         }
+        async fn list_paginated(&self, page: PageParams) -> Result<Paginated<UserSummary>, DomainError> {
+            self.0.list_paginated(page).await
+        }
+        async fn find_by_ids(&self, ids: &[UserId]) -> Result<std::collections::HashMap<UserId, User>, DomainError> {
+            self.0.find_by_ids(ids).await
+        }
     }
 
     #[async_trait]
@@ -177,6 +183,12 @@ mod tests {
         async fn count(&self) -> Result<i64, DomainError> {
             self.0.count().await
         }
+        async fn list_paginated(&self, page: PageParams) -> Result<Paginated<UserSummary>, DomainError> {
+            self.0.list_paginated(page).await
+        }
+        async fn find_by_ids(&self, ids: &[UserId]) -> Result<std::collections::HashMap<UserId, User>, DomainError> {
+            self.0.find_by_ids(ids).await
+        }
     }
 
     #[async_trait]

crates/application/src/use_cases/thoughts.rs

@@ -1,10 +1,14 @@
 use domain::{
     errors::DomainError,
     events::DomainEvent,
-    models::thought::{Thought, Visibility},
+    models::{
-    ports::{EventPublisher, OutboxWriter, TagRepository, ThoughtRepository, UserReader},
+        feed::{EngagementStats, FeedEntry},
+        thought::{Thought, Visibility},
+    },
+    ports::{EngagementRepository, EventPublisher, OutboxWriter, TagRepository, ThoughtRepository, UserReader},
     value_objects::{Content, ThoughtId, UserId},
 };
+use std::collections::HashMap;
 
 fn require_owner(thought: &Thought, user_id: &UserId) -> Result<(), DomainError> {
     if thought.user_id != *user_id {
@@ -127,6 +131,67 @@ pub async fn get_thread(
     thoughts.get_thread(id).await
 }
 
+/// Fetches a single thought enriched with author + real engagement stats.
+pub async fn get_thought_view(
+    thoughts: &dyn ThoughtRepository,
+    users: &dyn UserReader,
+    engagement: &dyn EngagementRepository,
+    id: &ThoughtId,
+    viewer: Option<&UserId>,
+) -> Result<FeedEntry, DomainError> {
+    let thought = thoughts
+        .find_by_id(id)
+        .await?
+        .ok_or(DomainError::NotFound)?;
+    let author = users
+        .find_by_id(&thought.user_id)
+        .await?
+        .ok_or(DomainError::NotFound)?;
+    let mut map = engagement.get_for_thoughts(&[id.clone()], viewer).await?;
+    let (stats, viewer_ctx) = map.remove(id).unwrap_or_else(|| {
+        (EngagementStats { like_count: 0, boost_count: 0, reply_count: 0 }, None)
+    });
+    Ok(FeedEntry { thought, author, stats, viewer: viewer_ctx })
+}
+
+/// Fetches a thread (root + replies) enriched with authors + real engagement stats.
+/// Batches all DB lookups — one query per resource type regardless of thread length.
+pub async fn get_thread_views(
+    thoughts: &dyn ThoughtRepository,
+    users: &dyn UserReader,
+    engagement: &dyn EngagementRepository,
+    root_id: &ThoughtId,
+    viewer: Option<&UserId>,
+) -> Result<Vec<FeedEntry>, DomainError> {
+    let thread = thoughts.get_thread(root_id).await?;
+    if thread.is_empty() {
+        return Ok(vec![]);
+    }
+
+    let thought_ids: Vec<ThoughtId> = thread.iter().map(|t| t.id.clone()).collect();
+    let user_ids: Vec<UserId> = thread.iter().map(|t| t.user_id.clone()).collect();
+
+    let (authors_map, engagement_map) = tokio::join!(
+        users.find_by_ids(&user_ids),
+        engagement.get_for_thoughts(&thought_ids, viewer),
+    );
+    let authors_map = authors_map?;
+    let mut engagement_map = engagement_map?;
+
+    let mut entries = Vec::with_capacity(thread.len());
+    for thought in thread {
+        let author = authors_map
+            .get(&thought.user_id)
+            .cloned()
+            .ok_or(DomainError::NotFound)?;
+        let (stats, viewer_ctx) = engagement_map.remove(&thought.id).unwrap_or_else(|| {
+            (EngagementStats { like_count: 0, boost_count: 0, reply_count: 0 }, None)
+        });
+        entries.push(FeedEntry { thought, author, stats, viewer: viewer_ctx });
+    }
+    Ok(entries)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -323,3 +388,84 @@ mod tests {
         assert_eq!(reply.in_reply_to_id, Some(original.id.clone()));
     }
 }
+
+#[cfg(test)]
+mod enrichment_tests {
+    use super::*;
+    use domain::testing::TestStore;
+    use domain::models::user::User;
+    use domain::models::thought::{Thought, Visibility};
+    use domain::value_objects::*;
+    use domain::ports::{ThoughtRepository, UserWriter};
+
+    fn make_user() -> User {
+        User::new_local(
+            UserId::new(),
+            Username::new("alice").unwrap(),
+            Email::new("a@a.com").unwrap(),
+            PasswordHash("h".into()),
+        )
+    }
+
+    fn make_thought(user_id: UserId) -> Thought {
+        Thought::new_local(
+            ThoughtId::new(),
+            user_id,
+            Content::new_local(String::from("hello")).unwrap(),
+            None,
+            Visibility::Public,
+            None,
+            false,
+        )
+    }
+
+    #[tokio::test]
+    async fn get_thought_view_returns_feed_entry() {
+        let store = TestStore::default();
+        let user = make_user();
+        <TestStore as UserWriter>::save(&store, &user).await.unwrap();
+        let thought = make_thought(user.id.clone());
+        <TestStore as ThoughtRepository>::save(&store, &thought).await.unwrap();
+
+        let entry = get_thought_view(&store, &store, &store, &thought.id, None)
+            .await
+            .unwrap();
+        assert_eq!(entry.thought.id, thought.id);
+        assert_eq!(entry.author.id, user.id);
+        assert_eq!(entry.stats.like_count, 0);
+        assert!(entry.viewer.is_none());
+    }
+
+    #[tokio::test]
+    async fn get_thought_view_returns_not_found_for_missing_thought() {
+        let store = TestStore::default();
+        let err = get_thought_view(&store, &store, &store, &ThoughtId::new(), None)
+            .await
+            .unwrap_err();
+        assert!(matches!(err, DomainError::NotFound));
+    }
+
+    #[tokio::test]
+    async fn get_thread_views_batches_correctly() {
+        let store = TestStore::default();
+        let user = make_user();
+        <TestStore as UserWriter>::save(&store, &user).await.unwrap();
+        let root = make_thought(user.id.clone());
+        <TestStore as ThoughtRepository>::save(&store, &root).await.unwrap();
+        let reply = Thought::new_local(
+            ThoughtId::new(),
+            user.id.clone(),
+            Content::new_local(String::from("reply")).unwrap(),
+            Some(root.id.clone()),
+            Visibility::Public,
+            None,
+            false,
+        );
+        <TestStore as ThoughtRepository>::save(&store, &reply).await.unwrap();
+
+        let entries = get_thread_views(&store, &store, &store, &root.id, None)
+            .await
+            .unwrap();
+        assert_eq!(entries.len(), 2);
+    }
+}