Add concept of "mergeable" activity posts

For example: when you rename a discussion, DiscussionRenamedPost is
created. If you rename it again immediately afterwards, then a new
DiscussionRenamedPost can be merged into the old one. This will either
result in the old one being updated with the new title, or it being
deleted all together if it was renamed back to the old title.

src/Core/Handlers/Events/DiscussionRenamedNotifier.php

@@ -27,7 +27,7 @@ class DiscussionRenamedNotifier
     {
         $post = $this->createPost($event);
 
-        $event->discussion->postWasAdded($post);
+        $event->discussion->addPost($post);
 
         if ($event->discussion->start_user_id !== $event->user->id) {
             $this->sendNotification($event, $post);
@@ -36,16 +36,12 @@ class DiscussionRenamedNotifier
 
     protected function createPost(DiscussionWasRenamed $event)
     {
-        $post = DiscussionRenamedPost::reply(
+        return DiscussionRenamedPost::reply(
             $event->discussion->id,
             $event->user->id,
             $event->oldTitle,
             $event->discussion->title
         );
-
-        $post->save();
-
-        return $post;
     }
 
     protected function sendNotification(DiscussionWasRenamed $event, DiscussionRenamedPost $post)

src/Core/Models/ActivityPost.php

@@ -0,0 +1,27 @@
+<?php namespace Flarum\Core\Models;
+
+abstract class ActivityPost extends Post implements MergeableInterface
+{
+    use MergeableTrait;
+
+    /**
+     * Unserialize the content attribute.
+     *
+     * @param  string  $value
+     * @return string
+     */
+    public function getContentAttribute($value)
+    {
+        return json_decode($value);
+    }
+
+    /**
+     * Serialize the content attribute.
+     *
+     * @param  string  $value
+     */
+    public function setContentAttribute($value)
+    {
+        $this->attributes['content'] = json_encode($value);
+    }
+}

src/Core/Models/Discussion.php

@@ -185,6 +185,24 @@ class Discussion extends Model
         $this->removedPosts[] = $post->id;
     }
 
+    public function addPost(Post $post)
+    {
+        if ($post instanceof MergeableInterface) {
+            $lastPost = $this->posts()->orderBy('time', 'desc')->first();
+            $post = $post->saveAfter($lastPost);
+        } else {
+            $post->save();
+        }
+
+        if ($post->exists) {
+            $this->postWasAdded($post);
+        } else {
+            $this->postWasRemoved($post);
+        }
+
+        return $post->exists;
+    }
+
     /**
      * Define the relationship with the discussion's posts.
      *

src/Core/Models/DiscussionRenamedPost.php

@@ -1,7 +1,23 @@
 <?php namespace Flarum\Core\Models;
 
-class DiscussionRenamedPost extends Post
+class DiscussionRenamedPost extends ActivityPost
 {
+    /**
+     * Merge the post into another post of the same type.
+     *
+     * @param \Flarum\Core\Models\DiscussionRenamedPost $previous
+     * @return boolean true if the post was merged, false if it was deleted.
+     */
+    protected function mergeInto(Model $previous)
+    {
+        if ($previous->content[0] == $this->content[1]) {
+            return false;
+        }
+
+        $previous->content = static::buildContent($previous->content[0], $this->content[1]);
+        return true;
+    }
+
     /**
      * Create a new instance in reply to a discussion.
      *
@@ -15,7 +31,7 @@ class DiscussionRenamedPost extends Post
     {
         $post = new static;
 
-        $post->content       = [$oldTitle, $newTitle];
+        $post->content       = static::buildContent($oldTitle, $newTitle);
         $post->time          = time();
         $post->discussion_id = $discussionId;
         $post->user_id       = $userId;
@@ -25,23 +41,14 @@ class DiscussionRenamedPost extends Post
     }
 
     /**
-     * Unserialize the content attribute.
+     * Build the content attribute.
      *
-     * @param  string  $value
-     * @return string
+     * @param boolean $oldTitle The old title of the discussion.
+     * @param boolean $newTitle The new title of the discussion.
+     * @return array
      */
-    public function getContentAttribute($value)
+    public static function buildContent($oldTitle, $newTitle)
     {
-        return json_decode($value);
-    }
-
-    /**
-     * Serialize the content attribute.
-     *
-     * @param  string  $value
-     */
-    public function setContentAttribute($value)
-    {
-        $this->attributes['content'] = json_encode($value);
+        return [$oldTitle, $newTitle];
     }
 }

src/Core/Models/MergeableInterface.php

@@ -0,0 +1,6 @@
+<?php namespace Flarum\Core\Models;
+
+interface MergeableInterface
+{
+    public function saveAfter(Model $previous);
+}

src/Core/Models/MergeableTrait.php

@@ -0,0 +1,23 @@
+<?php namespace Flarum\Core\Models;
+
+trait MergeableTrait
+{
+    public function saveAfter(Model $previous)
+    {
+        if ($previous instanceof static) {
+            if ($this->mergeInto($previous)) {
+                $previous->save();
+            } else {
+                $previous->delete();
+            }
+
+            return $previous;
+        }
+
+        $this->save();
+
+        return $this;
+    }
+
+    abstract protected function mergeInto(Model $previous);
+}