Alter previous method on all versions models

This is to prevent redoing the same SQL query which wasn't being cached.

app/models/post_archive.rb

@@ -92,13 +92,16 @@ class PostArchive < ApplicationRecord
   end
 
   def previous
-    # HACK: if all the post versions for this post have already been preloaded,
-    # we can use that to avoid a SQL query.
-    if association(:post).loaded? && post && post.association(:versions).loaded?
-      post.versions.sort_by(&:version).reverse.find { |v| v.version < version }
-    else
-      PostArchive.where("post_id = ? and version < ?", post_id, version).order("version desc").first
+    @previous ||= begin
+      # HACK: if all the post versions for this post have already been preloaded,
+      # we can use that to avoid a SQL query.
+      if association(:post).loaded? && post && post.association(:versions).loaded?
+        ver = [post.versions.sort_by(&:version).reverse.find { |v| v.version < version }]
+      else
+        ver = PostArchive.where("post_id = ? and version < ?", post_id, version).order("version desc").limit(1).to_a
+      end
     end
+    @previous.first
   end
 
   def visible?