Cleanup empty signatures

Some relayables from redmatrix and hubzilla have empty signatures. They
are invalid and break therefore the user data export.

closes #7644

Changelog.md

@@ -40,6 +40,7 @@ after you've upgraded.
 * 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)
+* Cleanup empty signatures [#7644](https://github.com/diaspora/diaspora/pull/7644)
 
 ## Features
 * Ask for confirmation when leaving a submittable comment field [#7530](https://github.com/diaspora/diaspora/pull/7530)

db/migrate/20171017221434_cleanup_empty_signatures.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+class CleanupEmptySignatures < ActiveRecord::Migration[5.1]
+  class Comment < ApplicationRecord
+    belongs_to :commentable, polymorphic: true
+
+    has_one :signature, class_name: "CommentSignature", dependent: :delete
+
+    before_destroy do
+      Like.where(target_type: "Comment", target_id: id).destroy_all
+      ActsAsTaggableOn::Tagging.where(taggable_type: "Comment", taggable_id: id).destroy_all
+    end
+
+    after_destroy do
+      commentable.update_comments_counter
+    end
+  end
+
+  class Like < ApplicationRecord
+    belongs_to :target, polymorphic: true
+
+    has_one :signature, class_name: "LikeSignature", dependent: :delete
+
+    after_destroy do
+      target.update_likes_counter
+    end
+  end
+
+  class PollParticipation < ApplicationRecord
+    belongs_to :poll_answer, counter_cache: :vote_count
+
+    has_one :signature, class_name: "PollParticipationSignature", dependent: :delete
+  end
+
+  def up
+    Comment.joins("INNER JOIN comment_signatures as signature ON comments.id = signature.comment_id")
+           .where("signature.author_signature = ''").destroy_all
+    Like.joins("INNER JOIN like_signatures as signature ON likes.id = signature.like_id")
+        .where("signature.author_signature = ''").destroy_all
+    PollParticipation.joins("INNER JOIN poll_participation_signatures as signature " \
+                            "ON poll_participations.id = signature.poll_participation_id")
+                     .where("signature.author_signature = ''").destroy_all
+  end
+end