Directly use ActiveRecord object instead of ID

This change will improve the performance by reducing unnecessary SELECT queries.

app/validators/code_ocean/file_name_validator.rb

@@ -3,8 +3,8 @@
 module CodeOcean
   class FileNameValidator < ActiveModel::Validator
     def validate(record)
-      existing_files = File.where(name: record.name, path: record.path, file_type_id: record.file_type_id,
-        context_id: record.context_id, context_type: record.context_type).to_a
+      existing_files = File.where(name: record.name, path: record.path, file_type: record.file_type,
+        context: record.context)
       if !existing_files.empty? && (!record.context.is_a?(Exercise) || record.context.new_record?)
         record.errors.add(:base, 'Duplicate')
       end