move model code out of migration

app/models/comment.rb

@@ -34,7 +34,8 @@ class Comment < ActiveRecord::Base
 
   # TODO: faking counter cache since there is a bug with acts_as_nested_set :counter_cache
   # Remove when https://github.com/collectiveidea/awesome_nested_set/issues/294 is fixed
-  # There is a test for this, so you should know if it is actually fixed.
+  # and reset counters using
+  # > Comment.find_each { |comment| Comment.reset_counters(comment.id, :children) }
   def children_count
     children.count
   end

db/migrate/20150811161459_add_children_count_to_comments.rb

@@ -1,10 +1,6 @@
 class AddChildrenCountToComments < ActiveRecord::Migration
   def up
     add_column :comments, :children_count, :integer, default: 0
-
-    Comment.find_each do |comment|
-      Comment.reset_counters(comment.id, :children)
-    end
   end
 
   def down