app-validator-ticket-grouper_validator.rb

class Ticket::GrouperValidator < ActiveModel::Validator
  def validate(record)
    record.errors.add :grouper, 'has already occurred!' if record.grouper.full?
    record.errors.add :grouper, 'has already occurred!' if record.grouper.past?
  end
end

app-validator-ticket-user_validator.rb

class Ticket::UserValidator < ActiveModel::Validator
  def validate(record)
    record.errors.add :user, "can't book a Grouper at this time" if record.user.blacklisted?
    record.errors.add :user, 'are already going to a Grouper on that day' if record.user.has_existing_grouper?(record.grouper)
  end
end

ticket.rb

class Ticket < ActiveRecord::Base
  belongs_to :grouper
  belongs_to :user

  validate confirmation_validations, on: :confirmation
  validate :ticket_cant_have_been_confirmed, on: :confirmation


  def confirm
    update confirmed: true if confirmable?
  end

  def confirmable?
    valid? :confirmation
  end

  private

  def confirmation_validations
    validates_with Ticket::UserValidator, Ticket::GrouperValidation
  end

  def ticket_cant_have_been_confirmed
    errors.add :user, 'have already confirmed this ticket' if confirmed?
  end
end