feat(db): Add view for ordering thread index by most recent post

This implements the classic thread ordering expected by most forum
users (i.e. the thread with the most recent post is at the top).

migrations/2018-04-14-145711_create_index_view/down.sql

@@ -0,0 +1 @@
+DROP VIEW thread_index;

migrations/2018-04-14-145711_create_index_view/up.sql

@@ -0,0 +1,15 @@
+-- Create a simple view that returns the list of threads ordered by
+-- the last post that occured in the thread.
+
+CREATE VIEW thread_index AS
+  SELECT t.id AS thread_id,
+         t.title AS title,
+         t.author_name AS author_name,
+         t.posted AS posted,
+         p.id AS post_id
+    FROM threads t
+    JOIN (SELECT DISTINCT ON (thread_id) id, thread_id
+          FROM posts
+          ORDER BY thread_id, id DESC) AS p
+    ON t.id = p.thread_id
+    ORDER BY p.id DESC;

src/db.rs

@@ -36,19 +36,18 @@ impl Actor for DbExecutor {
 pub struct ListThreads;
 
 impl Message for ListThreads {
-    type Result = Result<Vec<Thread>>;
+    type Result = Result<Vec<ThreadIndex>>;
 }
 
 impl Handler<ListThreads> for DbExecutor {
     type Result = <ListThreads as Message>::Result;
 
     fn handle(&mut self, _: ListThreads, _: &mut Self::Context) -> Self::Result {
-        use schema::threads::dsl::*;
+        use schema::thread_index::dsl::*;
 
         let conn = self.0.get()?;
-        let results = threads
-            .order(posted.desc())
-            .load::<Thread>(&conn)?;
+        let results = thread_index
+            .load::<ThreadIndex>(&conn)?;
         Ok(results)
     }
 }

src/models.rs

@@ -26,6 +26,18 @@ pub struct Thread {
     pub author_email: String,
 }
 
+/// This struct is used as the query type for the thread index view,
+/// which lists the index of threads ordered by the last post in each
+/// thread.
+#[derive(Queryable, Serialize)]
+pub struct ThreadIndex {
+    pub thread_id: i32,
+    pub title: String,
+    pub author_name: String,
+    pub posted: DateTime<Utc>,
+    pub post_id: i32,
+}
+
 #[derive(Identifiable, Queryable, Serialize, Associations)]
 #[belongs_to(Thread)]
 pub struct Post {

src/render.rs

@@ -47,7 +47,7 @@ impl From<DateTime<Utc>> for FormattedDate {
 
 /// Message used to render the index page.
 pub struct IndexPage {
-    pub threads: Vec<Thread>,
+    pub threads: Vec<ThreadIndex>,
 }
 
 impl Message for IndexPage {
@@ -69,7 +69,7 @@ impl Handler<IndexPage> for Renderer {
         let threads: Vec<IndexThread> = msg.threads
             .into_iter()
             .map(|thread| IndexThread {
-                id: thread.id,
+                id: thread.thread_id,
                 title: escape_html(&thread.title),
                 posted: thread.posted.into(),
                 author_name: thread.author_name,

src/schema.rs

@@ -19,6 +19,17 @@ table! {
     }
 }
 
+// Note: Manually inserted as print-schema does not add views.
+table! {
+    thread_index (thread_id){
+        thread_id -> Integer,
+        title -> Text,
+        author_name -> Text,
+        posted -> Timestamptz,
+        post_id -> Integer,
+    }
+}
+
 joinable!(posts -> threads (thread_id));
 
 allow_tables_to_appear_in_same_query!(