ActiveStorage File Attachment Validation

Question

Is there a way to validate attachments with ActiveStorage? For example, if I want to validate the content type or the file size?

Something like Paperclip\'s approach wou

Answer 1

Here is my solution to validate content types in Rails 5.2, that as you may know it has the pitfall that attachments are saved as soon as they are assigned to a model. It may also work for Rails 6. What I did is monkey-patch ActiveStorage::Attachment to include validations:

config/initializers/active_storage_attachment_validations.rb:

Rails.configuration.to_prepare do
  ActiveStorage::Attachment.class_eval do
    ALLOWED_CONTENT_TYPES = %w[image/png image/jpg image/jpeg].freeze

    validates :content_type, content_type: { in: ALLOWED_CONTENT_TYPES, message: 'of attached files is not valid' }
  end
end

app/validators/content_type_validator.rb:

class ContentTypeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, _value)
    return true if types.empty?
    return true if content_type_valid?(record)

    errors_options = { authorized_types: types.join(', ') }
    errors_options[:message] = options[:message] if options[:message].present?
    errors_options[:content_type] = record.blob&.content_type
    record.errors.add(attribute, :content_type_invalid, errors_options)
  end

  private

  def content_type_valid?(record)
    record.blob&.content_type.in?(types)
  end

  def types
    Array.wrap(options[:with]) + Array.wrap(options[:in])
  end
end

Due to the implementation of the attach method in Rails 5:

    def attach(*attachables)
      attachables.flatten.collect do |attachable|
        if record.new_record?
          attachments.build(record: record, blob: create_blob_from(attachable))
        else
          attachments.create!(record: record, blob: create_blob_from(attachable))
        end
      end
    end

The create! method raises an ActiveRecord::RecordInvalid exception when validations fail, but it just needs to be rescued and that's all.