Shopify Script, Buy Product from vendor X get Product Y for free

In shopify I was looking for a way to get a free product when buying from a specific vendor, I came up with below script.
In our case we also needed the vendor’s product to be at > $400 for the gift product to be free, you can offcourse remove that if statement.
Unfortunately you can’t add products to the cart with shopify scripts (yet?) so you can leave adding the product up to the customer, or add the product with javascript in the frontend.

You can remove the puts statements, I though I’d leave them so you can see how to debug something like this.

discounted_product = 2228740609
products_seen = false;

puts('loop over all products, and check if we have the required vendor')
Input.cart.line_items.each do |line_item|
  product = line_item.variant.product
  puts(product.vendor.downcase)
  if(product.vendor.downcase == 'vendorname')
    
    if(line_item.variant.price > Money.new(cents: 40000))
      puts('found a product matching vendor, and matching minimum price of $400')
      products_seen = true;
    end
  end
end

if(products_seen)
  puts('Check if the gift product is in the cart, and update its price to 0')
  Input.cart.line_items.each do |line_item|
    product = line_item.variant.product
    next unless product.id == discounted_product
      puts('they will only get 1 free, not unlimited items')
      updated_price = line_item.line_price.cents - (line_item.line_price.cents / line_item.quantity)
      line_item.change_line_price( Money.new(cents: updated_price) , message: "Free Shoetrees")
  end
end

Output.cart = Input.cart