Custom Validator on: :create not running on rails app - ruby-on-rails

Question

I have a rails application for creating volumes and have written two custom validators using ActiveModel::Validator.
volume.rb:
class Volume < ActiveRecord::Base
include UrlSafeCode
include PgSearch::Model
include ActiveModel::Validations
validates :user_id, presence: true
validates_with Validators::VolumeValidator
validates_with Validators::CreateVolumeValidator, on: :create
def self.digest text
Digest::SHA256.hexdigest(text)
end
def text=(new_text)
new_text.rstrip!
new_text.downcase!
self.text_digest = Volume.digest(new_text)
super(new_text)
end
My Problem: The CreateVolumeValidator checks if a record with the same text_digest is already in the database. I only want to run this when creating a new volume so that I can still update existing volumes. However, adding on: :create to the CustomVolumeValidator causes the validator to stop working.
I've read through a lot of the other entries about similar issues and haven't found a solution. I am pretty sure I am missing something about when different attributes are getting created, validated, and saved, but I haven't worked with custom validations much, and I'm lost.
Here is the other relevant code.
volumes_controller.rb
def new
#volume = Volume.new
end
def create
our_params = params
.permit(:text, :description)
if params[:text].nil?
render :retry
return
end
text = params[:text].read.to_s
text_digest = Volume.digest(text)
#description = our_params[:description]
begin
#volume = Volume.where(text_digest: text_digest)
.first_or_create(text: text, user: current_user, description: our_params[:description])
rescue ActiveRecord::RecordNotUnique
retry
end
if #volume.invalid?
render :retry
return
end
render :create
end
def edit
get_volume
end
def update
get_volume
unless #volume
render nothing: true, status: :not_found
return
end
#volume.update(params.require(:volume).permit(:text, :description))
if #volume.save
redirect_to volume_path(#volume.code)
else
flash[:notice] = #volume.errors.full_messages.join('\n')
render :edit
end
end
def get_volume
#volume = Volume.where(code: params.require(:code)).first
end
create_volume_validator.rb
class Validators::CreateVolumeValidator < ActiveModel::Validator
def validate(volume)
existing_volume = Volume.where(text_digest: volume.text_digest).first
if existing_volume
existing_volume_link = "<a href='#{Rails.application.routes.url_helpers.volume_path(existing_volume.code)}'>here</a>."
volume.errors.add :base, ("This volume is already part of the referral archive and is available " + existing_volume_link).html_safe
end
end
end

Answer 1

If your goal is for all Volume records to have unique text_digest, you are better off with a simple :uniqueness validator (and associated DB unique index).
However, the reason your existing code isn't working is:
Volume.where(text_digest: text_digest).first_or_create(...)
This returns either the first Volume with the matching text_digest or creates a new one. But that means if there is a conflict, no object is created, and therefore your (on: :create) validation doesn't run. Instead, it simply sets #volume to the existing object, which is, by definition, valid. If there is no matching record, it does call your validator, but there's nothing to validate because you've already proved there is no text_digest conflict.
You could resolve by replacing the first_or_create with create, but again, you are vastly better off with a unique index & validator (with custom message if you like).