spree/spree_related_products

module Spree
  class Calculator::RelatedProductDiscount < Spree::Calculator
    def self.description
      Spree.t(:related_product_discount)
    end

    def compute(object)
      if object.is_a?(Array)
        return if object.empty?
        order = object.first.order
      else
        order = object
      end

      total = 0
      if eligible?(order)
        total = order.line_items.inject(0) do |sum, line_item|
          relations =  Spree::Relation.where(*discount_query(line_item))
          discount_applies_to = relations.map {|rel| rel.related_to.master.product }
          order.line_items.each do |li|
            next unless discount_applies_to.include? li.variant.product
            discount = relations.detect { |rel| rel.related_to.master.product == li.variant.product }.discount_amount
            sum +=  if li.quantity < line_item.quantity
                      (discount * li.quantity)
                    else
                      (discount * line_item.quantity)
                    end
          end

          sum
        end
      end

      total
    end

    def eligible?(order)
      order.line_items.any? do |line_item|
        Spree::Relation.exists?(discount_query(line_item))
      end
    end

    def discount_query(line_item)
      [
        'discount_amount <> 0.0 AND relatable_type = ? AND relatable_id = ?',
        'Spree::Product',
        line_item.variant.product.id
      ]
    end
  end
end