A validator and RSpec matcher for requiring an attachment using Active Storage

attached_validator.rb

class AttachedValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    record.errors.add(attribute, :attached, options) unless value.attached?
  end
end

en.yml

en:
  activerecord:
    errors:
      messages:
        attached: is not attached

validate_attachment_of.rb

require "rspec/expectations"

RSpec::Matchers.define :validate_attachment_of do |attr_name|
  match do |record| 
    matcher.matches?(record, attr_name)
  end

  chain :on do |validation_context|
    matcher.on(validation_context)
  end

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

  private

  def matcher
    @matcher ||= ValidateAttachmentOfMatcher.new
  end

  class ValidateAttachmentOfMatcher
    def on(validation_context)
      @validation_context = validation_context
    end

    def with_message(message)
      @message = message
    end

    def matches?(record, attr_name)
      record.send(attr_name).purge
      record.valid?(validation_context)
      record.errors[attr_name].include? message
    end
    
    private

    attr_reader :validation_context

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

Thanks for putting this together. This has worked great for me in Rails 5.2. In Rails 6, you will get an FrozenError: can't modify frozen Hash error from line 32. Instead of purging, the blob should be set to nil.

To fix, that line should be replaced with:

record.send("#{attr_name}=", nil)

An explanation here: rails/rails#37069