diff --git a/framework/core/src/Database/InsertsViaSubqueryTrait.php b/framework/core/src/Database/InsertsViaSubqueryTrait.php
new file mode 100644
index 000000000..78d57418c
--- /dev/null
+++ b/framework/core/src/Database/InsertsViaSubqueryTrait.php
@@ -0,0 +1,73 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Database;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\MySqlConnection;
+
+trait InsertsViaSubqueryTrait
+{
+    /**
+     * A list that maps attribute names to callables that produce subqueries
+     * used to calculate the inserted value.
+     *
+     * Each callable should take an instance of the model being saved,
+     * and return an Eloquent query builder that queries for the subquery
+     * generated value. The result of the query should be one row of one column.
+     *
+     * Subquery attributes should be added in the static `boot` method of models
+     * using this trait.
+     *
+     * @var array<string, callable(AbstractModel): Builder>
+     */
+    protected static $subqueryAttributes = [];
+
+    /**
+     * Overriden so that some fields can be inserted via subquery.
+     * The goal here is to construct a subquery that returns primitives
+     * for the provided `attributes`, and uses additional subqueries for
+     * statically-specified subqueryAttributes.
+     */
+    protected function insertAndSetId(Builder $query, $attributes)
+    {
+        $subqueryAttrNames = array_keys(static::$subqueryAttributes);
+
+        $literalAttributes = array_diff_key($attributes, array_flip($subqueryAttrNames));
+
+        /** @var Builder */
+        $insertRowSubquery = static::query()->limit(1);
+
+        foreach ($literalAttributes as $attrName => $value) {
+            $parameter = $query->getGrammar()->parameter($value);
+            $insertRowSubquery->addBinding($value, 'select');
+            $insertRowSubquery->selectRaw("$parameter as $attrName");
+        }
+
+        foreach (static::$subqueryAttributes as $attrName => $callback) {
+            $insertRowSubquery->selectSub($callback($this), $attrName);
+        }
+
+        $attrNames = array_merge(array_keys($literalAttributes), $subqueryAttrNames);
+        $query->insertUsing($attrNames, $insertRowSubquery);
+
+        // This should be accurate, as it's the same mechanism used by Laravel's `insertGetId`.
+        // See https://github.com/laravel/framework/blob/master/src/Illuminate/Database/Query/Processors/Processor.php#L30-L37.
+        /** @var MySqlConnection */
+        $con = $query->getQuery()->getConnection();
+        $idRaw = $con->getPdo()->lastInsertId($keyName = $this->getKeyName());
+        $id = is_numeric($idRaw) ? (int) $idRaw : $idRaw;
+
+        $this->setAttribute($keyName, $id);
+
+        // This is necessary to get the computed value of saved attributes.
+        $this->exists = true;
+        $this->refresh();
+    }
+}
diff --git a/framework/core/src/Post/Command/PostReplyHandler.php b/framework/core/src/Post/Command/PostReplyHandler.php
index 40a52de7f..96de698d0 100644
--- a/framework/core/src/Post/Command/PostReplyHandler.php
+++ b/framework/core/src/Post/Command/PostReplyHandler.php
@@ -74,7 +74,7 @@ class PostReplyHandler
 
         // If this is the first post in the discussion, it's technically not a
         // "reply", so we won't check for that permission.
-        if ($discussion->post_number_index > 0) {
+        if ($discussion->first_post_id !== null) {
             $actor->assertCan('reply', $discussion);
         }
 
diff --git a/framework/core/src/Post/Post.php b/framework/core/src/Post/Post.php
index f9deff3d2..fae314772 100644
--- a/framework/core/src/Post/Post.php
+++ b/framework/core/src/Post/Post.php
@@ -10,6 +10,7 @@
 namespace Flarum\Post;
 
 use Flarum\Database\AbstractModel;
+use Flarum\Database\InsertsViaSubqueryTrait;
 use Flarum\Database\ScopeVisibilityTrait;
 use Flarum\Discussion\Discussion;
 use Flarum\Foundation\EventGeneratorTrait;
@@ -41,6 +42,7 @@ class Post extends AbstractModel
 {
     use EventGeneratorTrait;
     use ScopeVisibilityTrait;
+    use InsertsViaSubqueryTrait;
 
     protected $table = 'posts';
 
@@ -102,6 +104,10 @@ class Post extends AbstractModel
         });
 
         static::addGlobalScope(new RegisteredTypesScope);
+
+        static::$subqueryAttributes['number'] = function (Post $post) {
+            return static::query()->where('discussion_id', $post->discussion_id)->selectRaw('max(number)+1');
+        };
     }
 
     /**