Update rails and code

app/models/admin.rb

@@ -1,4 +1,4 @@
-class Admin < ActiveRecord::Base
+class Admin < ApplicationRecord
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable and :omniauthable
   devise :database_authenticatable,

app/models/application_record.rb

@@ -0,0 +1,3 @@
+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end

app/models/problem.rb

@@ -1,6 +1,6 @@
-class Problem < ActiveRecord::Base
+class Problem < ApplicationRecord
   # TODO Check whether there is a better relation for this
-  belongs_to :translation
+  belongs_to :translation, optional: true
   delegate :title, :content, to: :translation
 
   has_many :translations, inverse_of: :problem

app/models/translation.rb

@@ -1,4 +1,4 @@
-class Translation < ActiveRecord::Base
+class Translation < ApplicationRecord
   belongs_to :problem, inverse_of: :translations
 
   validates :title, :content, :problem_id, presence: true