Clean up invalid contacts from the DB

Cleans invalid contacts where the referenced user was deleted from the DB or where
the referenced person owner was deleted from the DB.

closes #7632

Changelog.md

@@ -39,6 +39,7 @@ after you've upgraded.
 * Avoid page to jump to top after a post deletion [#7638](https://github.com/diaspora/diaspora/pull/7638)
 * Handle duplicate account deletions [#7639](https://github.com/diaspora/diaspora/pull/7639)
 * Handle duplicate account migrations [#7641](https://github.com/diaspora/diaspora/pull/7641)
+* Handle bugs related to missing users [#7632](https://github.com/diaspora/diaspora/pull/7632)
 
 ## Features
 * Ask for confirmation when leaving a submittable comment field [#7530](https://github.com/diaspora/diaspora/pull/7530)

app/models/notification.rb

@@ -6,7 +6,7 @@
 #
 class Notification < ApplicationRecord
   belongs_to :recipient, class_name: "User"
-  has_many :notification_actors, dependent: :destroy
+  has_many :notification_actors, dependent: :delete_all
   has_many :actors, class_name: "Person", through: :notification_actors, source: :person
   belongs_to :target, polymorphic: true
 

db/migrate/20171012202650_cleanup_invalid_contacts.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+class CleanupInvalidContacts < ActiveRecord::Migration[5.1]
+  class Contact < ApplicationRecord
+    belongs_to :user
+    belongs_to :person
+
+    has_many :aspect_memberships, dependent: :delete_all
+
+    before_destroy :destroy_notifications
+
+    def destroy_notifications
+      Notification.where(
+        target_type:  "Person",
+        target_id:    person_id,
+        recipient_id: user_id,
+        type:         "Notifications::StartedSharing"
+      ).destroy_all
+    end
+  end
+
+  class User < ApplicationRecord
+  end
+
+  class Person < ApplicationRecord
+    belongs_to :owner, class_name: "User", optional: true
+  end
+
+  class Notification < ApplicationRecord
+    self.inheritance_column = nil
+    has_many :notification_actors, dependent: :delete_all
+  end
+
+  class NotificationActor < ApplicationRecord
+  end
+
+  class AspectMembership < ApplicationRecord
+  end
+
+  def up
+    Contact.left_outer_joins(:user).where("users.id is NULL").destroy_all
+    Contact.left_outer_joins(person: :owner).where("people.owner_id is NOT NULL").where("users.id is NULL").destroy_all
+  end
+end