Validate that a `Page` does not have any siblings with the same `:slug`

matching_page_slug_validator.rb

class MatchingPageSlugValidator < ActiveModel::Validator
  def validate(record)
    # Ancestry gem #siblings includes current record, exclude this if present
    exclude_self = record.try(:id) ? ['id <> ?', record.id] : []

    sibling_slugs = record.siblings.where(exclude_self).pluck(:slug)
    if sibling_slugs.include?(record.slug)
      record.errors[:slug] << 'belongs to another page at this level.'
    end
  end
end