Delete only the FOLLOWING tasks.

app/models/periodic_task_group.rb

@@ -1,23 +1,29 @@
 class PeriodicTaskGroup < ActiveRecord::Base
-  has_many :tasks, :dependent => :destroy
+  has_many :tasks, dependent: :destroy
 
   PeriodDays = 7
   
   def has_next_task?
-    return false if self.tasks.empty?
-    return false if self.tasks.first.due_date.nil?
+    return false if tasks.empty?
+    return false if tasks.first.due_date.nil?
     return true
   end
 
   def create_next_task
-    template_task = self.tasks.first
+    template_task = tasks.first
     self.next_task_date ||= template_task.due_date + PeriodDays
     
     next_task = template_task.dup
-    next_task.due_date = self.next_task_date    
+    next_task.due_date = next_task_date    
     next_task.save
 
     self.next_task_date += PeriodDays
     self.save
   end
+
+  def exclude_tasks_before(task)
+    tasks.where("due_date < '#{task.due_date}'").each do |t|
+      t.update_attribute(:periodic_task_group, nil)
+    end
+  end
 end