diff --git a/boards/models/post/__init__.py b/boards/models/post/__init__.py
--- a/boards/models/post/__init__.py
+++ b/boards/models/post/__init__.py
@@ -146,18 +146,7 @@ class Post(models.Model, Viewable):
     def get_absolute_url(self, thread=None):
         # Url is cached only for the "main" thread. When getting url
         # for other threads, do it manually.
-        url = self.url
-
-        if url is None:
-            if thread is None:
-                thread = self.get_thread()
-            opening = self.is_opening()
-            opening_id = self.id if opening else thread.get_opening_post_id()
-            url = reverse('thread', kwargs={'post_id': opening_id})
-            if not opening:
-                url += '#' + str(self.id)
-
-        return url
+        return self.url
 
     def get_thread(self):
         return self.thread
@@ -309,9 +298,14 @@ class Post(models.Model, Viewable):
                 except ObjectDoesNotExist:
                     pass
 
-    def build_url(self):
-        self.url = self.get_absolute_url()
-        self.save(update_fields=['url'])
+    def _build_url(self):
+        opening = self.is_opening()
+        opening_id = self.id if opening else self.get_thread().get_opening_post_id()
+        url = reverse('thread', kwargs={'post_id': opening_id})
+        if not opening:
+            url += '#' + str(self.id)
+
+        return url
 
     def save(self, force_insert=False, force_update=False, using=None,
              update_fields=None):
@@ -329,8 +323,9 @@ class Post(models.Model, Viewable):
 
         super().save(force_insert, force_update, using, update_fields)
 
-        if self.url is None:
-            self.build_url()
+        if new_post:
+            self.url = self._build_url()
+            super().save(update_fields=['url'])
 
     def get_text(self) -> str:
         return self._text_rendered