FIX: Comment serialization for deleted users

[s3lase]

Serializing topics with deleted comment users results in unhandled errors.

This change guards the user details access on QuestionAnswerComment records and also cleans up post voting related records when a user is destroyed via UserDestroyer.

[jjaffeux]

Did you consider using some null object pattern here?

# frozen_string_literal: true

module DiscoursePostVoting
  class DeletedCommentUser < User
    def username
      I18n.t("discourse_post_voting.deleted_user_username")
    end
    
    def name
      I18n.t("discourse_post_voting.deleted_user_name")
    end

    def avatar_template
      "/plugins/discourse-post-voting/images/deleted-user-avatar.png"
    end
  end
end

class QuestionAnswerCommentSerializer < ApplicationSerializer
  def user
    object.user || DiscoursePostVoting::DeletedCommentUser.new
  end
  
  def username
    user.username
  end
  
  def name
    user.name
  end
end

This might not suit your use case, but it's an easy solution to have a system working in every cases without having to clean the DB and maintaining state. eg: should we really remove the votes of a deleted user?

[s3lase]

Did you consider using some null object pattern here?

Yeah. I considered that but tabled it for much later when we have more cases of details of possibly deleted users being accessed. Returning a null user's username would have required additional plumbing work in core to have user cards work with a null user's usernames.

This might not suit your use case, but it's an easy solution to have a system working in every cases without having to clean the DB and maintaining state. eg: should we really remove the votes of a deleted user?

I say yes. Looking at UserDestroy we seem to delete a bunch of other records associated with the user. The vote deletion is an extension of that.

[s3lase]

thanks @ZogStriP