Use hash instead of json.

app/models/audit_event.rb

 class AuditEvent < ActiveRecord::Base
-  serialize :details, JSON
+  serialize :details, Hash
 
   belongs_to :user, foreign_key: :author_id
 

app/views/audit_events/_event_table.html.haml

@@ -23,7 +23,7 @@
           %td
           %td #{raw human_text(event.details)}
           %td
-          %td #{event.details['target_details']}
+          %td #{event.details[:target_details]}
           %td
           %td #{event.created_at}
       = paginate events