I have created a function to get all possible promotions of a product by considering even the taxons. Its not yet optimised and not well tested. I added the following in the product decorator model

gistfile1.txt

    def get_all_possible_promotions
        taxons_of_product_including_parents
    end


     private



            # ids of taxons rules and taxons rules children
            def taxons_including_children_ids
              promotion_rules_taxons = Spree::Taxon.where("id IN (?)", Spree::PromotionRuleTaxon.select(:taxon_id).where("True") )
              promotion_rules_taxons.inject([]) { |ids, taxon| ids += taxon.self_and_descendants.ids }
            end

            # taxons order vs taxons rules and taxons rules children
            def product_taxons_in_taxons_and_children()
              self.taxons.where(id: taxons_including_children_ids)
            end

            def taxons_of_product_including_parents()
              eligible_taxons = product_taxons_in_taxons_and_children().inject([]) { |taxons, taxon| taxons << taxon.self_and_ancestors }.flatten.uniq
                final_taxon_promotions =  eligible_taxons.inject([]) { |promotions,taxon|
                  promotions += taxon.promotion_rules.inject([]){|promotion,promotion_rule|

                   promotion << promotion_rule.promotion  if ! promotion_rule.promotion.expired?
                  }

                 }.flatten.uniq 
                 final_taxon_promotions += self.possible_promotions
                 final_taxon_promotions.uniq 
            end