altered class: to class_name: (class seems to be deprecated?)

app/models/comment.rb

@@ -3,6 +3,6 @@ class Comment < ActiveRecord::Base
   include Creation
   attr_accessor :username
 
-  belongs_to :file, class: CodeOcean::File
+  belongs_to :file, class_name: 'CodeOcean::File'
   belongs_to :user, polymorphic: true
 end

app/models/concerns/context.rb

@@ -2,7 +2,7 @@ module Context
   extend ActiveSupport::Concern
 
   included do
-    has_many :files, as: :context, class: CodeOcean::File
+    has_many :files, as: :context, class_name: 'CodeOcean::File'
     accepts_nested_attributes_for :files
   end
 

app/models/request_for_comment.rb

@@ -1,6 +1,6 @@
 class RequestForComment < ActiveRecord::Base
   belongs_to :exercise
-  belongs_to :file, class: CodeOcean::File
+  belongs_to :file, class_name: 'CodeOcean::File'
   belongs_to :user, polymorphic: true
 
   before_create :set_requested_timestamp