fixes #1970

app/models/post.rb

@@ -18,6 +18,7 @@ class Post < ActiveRecord::Base
   before_validation :initialize_uploader, :on => :create
   before_validation :parse_pixiv_id
   before_validation :blank_out_nonexistent_parents
+  before_validation :remove_parent_loops
   belongs_to :updater, :class_name => "User"
   belongs_to :approver, :class_name => "User"
   belongs_to :uploader, :class_name => "User"
@@ -814,6 +815,13 @@ class Post < ActiveRecord::Base
       end
     end
 
+    def remove_parent_loops
+      if parent.present? && parent.parent_id == id
+        parent.parent_id = nil
+        parent.save
+      end
+    end
+
     def validate_parent_does_not_have_a_parent
       return if parent.nil?
       if !parent.parent.nil?