Fix missing/incorrect Model specifications in migrations

We need to define the required models within the migration (not below) to work reliably.

db/migrate/20140918063522_add_hashed_content_to_files.rb

@@ -1,6 +1,20 @@
 # frozen_string_literal: true
 
 class AddHashedContentToFiles < ActiveRecord::Migration[4.2]
+  class CodeOcean::File < ApplicationRecord
+    before_validation :hash_content, if: :content_present?
+
+    private
+
+    def content_present?
+      content? || native_file?
+    end
+
+    def hash_content
+      self.hashed_content = Digest::MD5.new.hexdigest(read || '')
+    end
+  end
+
   def change
     add_column :files, :hashed_content, :string