Specify correct default values for events

app/models/event.rb

@@ -29,6 +29,10 @@ class Event < ActiveRecord::Base
     proposer.personal_profile(conference)
   end
 
+  def length
+    read_attribute(:length) || event_type.try(:minimum_length)
+  end
+
   private
 
   def track_belongs_to_the_selected_conference

db/migrate/20150819213202_drop_default_value_for_length_in_events.rb

@@ -0,0 +1,5 @@
+class DropDefaultValueForLengthInEvents < ActiveRecord::Migration
+  def change
+    change_column_default(:events, :length, nil)
+  end
+end