FIX: old migration was loading up invalid model schema

Generally we should never be touching AR objects in migrations, this is
super risky as we may end up with invalid schema cache.

This code from 2013 did it unconditionally. This change amends it so:

1. We only load up schema if we have no choice
2. We flush the cache before and after

This makes this migration far less risky.

db/migrate/20130221215017_add_description_to_categories.rb

@@ -8,10 +8,20 @@ class AddDescriptionToCategories < ActiveRecord::Migration[4.2]
     remove_column :categories, :top1_user_id
     remove_column :categories, :top2_user_id
 
-    # Migrate excerpts over
+    # some ancient installs may have bad category descriptions
-    Category.order('id').each do |c|
+    # attempt to fix
-      post = c.topic.posts.order(:post_number).first
+    if !DB.query_single("SELECT 1 FROM categories limit 1").empty?
-      PostRevisor.new(post).send(:update_category_description)
+
+      # Reaching into post revisor is not ideal here, but this code
+      # should almost never run, so bypass it
+      Discourse.reset_active_record_cache
+
+      Category.order('id').each do |c|
+        post = c.topic.ordered_posts.first
+        PostRevisor.new(post).update_category_description
+      end
+
+      Discourse.reset_active_record_cache
     end
 
   end