task: user dependent destroy

app/models/user.rb

@@ -9,7 +9,7 @@ class User < ApplicationRecord
   has_many :joined_user_activities
   has_many :activities, :through => :joined_user_activities
 
-  has_many :tasks, foreign_key: 'owner_id', class_name: 'Task'
+  has_many :tasks, foreign_key: 'owner_id', class_name: 'Task', dependent: :destroy
   
   has_many :memberships, foreign_key: :member_id, dependent: :destroy
   has_many :teams, through: :memberships