A validator and RSpec matcher for restricting an attachment’s content type using Active Storage

allow_content_type.rb

require "rspec/expectations"

RSpec::Matchers.define :allow_content_type do |*content_types|
  match do |record| 
    matcher.matches?(record, content_types)
  end

  chain :for do |attr_name|
    matcher.for(attr_name)
  end

  chain :with_message do |message|
    matcher.with_message(message)
  end

  private

  def matcher
    @matcher ||= AllowContentTypeMatcher.new
  end

  class AllowContentTypeMatcher
    def for(attr_name)
      @attr_name = attr_name
    end

    def with_message(message)
      @message = message
    end
      
    def matches?(record, content_types)
      Array.wrap(content_types).all? do |content_type|
        record.send(attr_name).attach attachment_for(content_type)
        record.valid?
        !record.errors[attr_name].include? message
      end
    end

    private

    attr_reader :attr_name

    def attachment_for(content_type)
      suffix = content_type.to_s.split("/").last

      { io: StringIO.new("Hello world!"), filename: "test.#{suffix}", content_type: content_type }
    end

    def message
      @message || I18n.translate("activerecord.errors.messages.content_type")
    end
  end
end

RSpec::Matchers.alias_matcher :allow_content_types, :allow_content_type

content_type_validator.rb

class ContentTypeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value.attached? && value.content_type.in?(content_types)
      value.purge if record.new_record? # Only purge the offending blob if the record is new
      record.errors.add(attribute, :content_type, options)
    end
  end

  private

  def content_types
    options.fetch(:in)
  end
end

en.yml

en:
  activerecord:
    errors:
      messages:
        content_type: is not a valid file format

Nice! How come you only purge the photo if it's a new record?

Hi! Thank you!
I don't understand much in Rails, so please excuse me if the question is lame.
I watch table — active_storage_blobs — and records are not deleted (active_storage_attachments is OK).
Am I doing something wrong?

This is great, thank you!

I modified this slightly for my use case such that it will allow the attachment to be nil.

class ContentTypeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return unless value.attached?
    return if value.content_type.in?(content_types)

    value.purge
    record.errors.add(attribute, :content_type, options)
  end

  private

  def content_types
    options.fetch(:in)
  end
end

Where (directory structure) in rails would I store this validator?

I get FrozenError: can't modify frozen Hash. Not sure where it's coming from.

@fydelio I store my validators in app/validators (i.e. app/validators/content_type_validator.rb)

Nice! How come you only purge the photo if it's a new record?

Great question! If a persisted record has an existing attachment then we don't want to purge it on a validation error because then we'll lose the existing value. I believe this behavior will be different in Rails 6+ because attachments are no longer persisted immediately upon attachment.

I watch table — active_storage_blobs — and records are not deleted (active_storage_attachments is OK).

If you call destroy on an attachment then it won't automatically delete the associated blob. You'll want to make sure you use the purge method for deleting the attachment and blob.

Where (directory structure) in rails would I store this validator?

@fydelio I store my validators in app/validators (i.e. app/validators/content_type_validator.rb)

This is exactly the directory structure I use as well!

I modified this slightly for my use case such that it will allow the attachment to be nil.

Awesome!

I get FrozenError: can't modify frozen Hash. Not sure where it's coming from.

@thelucid Yes, I got a similar error with the @meara approach. can't modify frozen attributes - Were you able to fix it? I added the check back and it worked as well:

def validate_each(record, attribute, value)

    return unless value.attached?
    return if value.content_type.in?(content_types)

    value.purge if record.new_record?
    record.errors.add(attribute, :content_type, **options)
  end

@carlosramireziii Thank you, it works nicely.