code: def collection_site
        return unless medical_identifiers.size == 1
        site = medical_identifiers.first.site
        return if site.nil?
        site.site_type == Site::SiteType::COLLECTION ? site : nil
      end
docs: Returns the collection site for which this participant has a MRN. If there is not exactly one
   such site, then this method returns nil. This method is a convenience for the common situation
   where a participant is enrolled at one site.
  
   @return [CaTissue::Site] the collection site