diff --git a/db/migrate/20121003223358_extract_backtraces.rb b/db/migrate/20121003223358_extract_backtraces.rb
index 388f905..f518cf2 100644
--- a/db/migrate/20121003223358_extract_backtraces.rb
+++ b/db/migrate/20121003223358_extract_backtraces.rb
@@ -1,11 +1,11 @@
 class ExtractBacktraces < Mongoid::Migration
   def self.up
     say "It could take long time (hours if you have many Notices)"
-    Notice.unscoped.all.each do |notice|
+    Notice.unscoped.where(backtrace_id: nil).each do |notice|
       next if notice.backtrace.present? || notice['backtrace'].nil?
       backtrace = Backtrace.find_or_create(:raw => notice['backtrace'] || [])
+      notice.unset(:backtrace)
       notice.backtrace = backtrace
-      notice['backtrace'] = nil
       notice.save!
     end
     say "run `db.repairDatabase()` (in mongodb console) to recover deleted space"
@@ -13,4 +13,4 @@ class ExtractBacktraces < Mongoid::Migration
 
   def self.down
   end
-end
\ No newline at end of file
+end
--
libgit2 0.21.2