Adopt Rails 5 belongs_to_required_by_default

plugins/links/app/models/link.rb

@@ -1,5 +1,5 @@
 class Link < ApplicationRecord
-  belongs_to :workgroup
+  belongs_to :workgroup, optional: true
 
   validates_presence_of :name, :url
 

plugins/messages/app/models/message.rb

@@ -1,9 +1,9 @@
 require "base32"
 
 class Message < ApplicationRecord
-  belongs_to :sender, :class_name => "User", :foreign_key => "sender_id"
-  belongs_to :group, :class_name => "Group", :foreign_key => "group_id"
-  belongs_to :reply_to_message, :class_name => "Message", :foreign_key => "reply_to"
+  belongs_to :sender, class_name: 'User', foreign_key: 'sender_id'
+  belongs_to :group, optional: true, class_name: 'Group', foreign_key: 'group_id'
+  belongs_to :reply_to_message, optional: true, class_name: 'Message', foreign_key: 'reply_to'
   has_many :message_recipients, dependent: :destroy
   has_many :recipients, through: :message_recipients, source: :user
 

plugins/polls/app/models/poll_vote.rb

@@ -1,6 +1,6 @@
 class PollVote < ActiveRecord::Base
   belongs_to :poll
-  belongs_to :ordergroup
+  belongs_to :ordergroup, optional: true
   belongs_to :user
   has_many :poll_choices, dependent: :destroy
 end

plugins/printer/app/models/printer_job.rb

@@ -2,7 +2,7 @@ class PrinterJob < ActiveRecord::Base
 
   belongs_to :order
   belongs_to :created_by, class_name: 'User', foreign_key: 'created_by_user_id'
-  belongs_to :finished_by, class_name: 'User', foreign_key: 'finished_by_user_id'
+  belongs_to :finished_by, optional: true, class_name: 'User', foreign_key: 'finished_by_user_id'
   has_many :printer_job_updates
 
   scope :finished, -> { where.not(finished_at: nil) }