ActiveStorage: How to validate content of file?

Question

I would like to read the content of a file that is to be uploaded in a custom validation. Is it possible?

So far, I've seen that it's possible to perform some validations on the file's metadata (extention, size...) but not on it's content. I've also seen this question that's a bit similar, even though I don't want to modify the file. I just want to read it in a custom validation.

Answer 1

It seems like you can access the file being uploaded and simply read it in the validation.

For Example

class User < ApplicationRecord
  has_one_attached :resume
  
  validate :resume_contents

  private 
    def resume_contents
      return unless record.send(attribute).attached? && !record.attachment_changes['resume'].blank?
      file = record.attachment_changes['resume'].attachable
      if file.is_a?(ActionDispatch::Http::UploadedFile) && file.read =~ /bad content/i
        errors.add(:resume, :invalid, message: 'cannot contain bad content')
      end
    end
end

For a more in depth implementation you could refer to the active_storage_validations gem:

• File Access
• File Reading