Destroy related todos when an issue is deleted

app/models/concerns/issuable.rb

@@ -19,6 +19,7 @@ module Issuable
     has_many :notes, as: :noteable, dependent: :destroy
     has_many :label_links, as: :target, dependent: :destroy
     has_many :labels, through: :label_links
+    has_many :todos, as: :target, dependent: :destroy
 
     validates :author, presence: true
     validates :title, presence: true, length: { within: 0..255 }

db/migrate/20160324020319_remove_todos_for_deleted_issues.rb

+class RemoveTodosForDeletedIssues < ActiveRecord::Migration
+  def up
+    execute <<-SQL
+      DELETE FROM todos
+      WHERE todos.target_type = 'Issue'
+        AND NOT EXISTS (
+              SELECT *
+              FROM issues
+              WHERE issues.id = todos.target_id
+                AND issues.deleted_at IS NULL
+            )
+    SQL
+  end
+
+  def down
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160320204112) do
+ActiveRecord::Schema.define(version: 20160324020319) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

spec/models/concerns/issuable_spec.rb

@@ -9,6 +9,7 @@ describe Issue, "Issuable" do
     it { is_expected.to belong_to(:author) }
     it { is_expected.to belong_to(:assignee) }
     it { is_expected.to have_many(:notes).dependent(:destroy) }
+    it { is_expected.to have_many(:todos).dependent(:destroy) }
   end
 
   describe "Validation" do