add_to_cart.js.erb

$('<%= j render partial: "cart_form", locals: {product: @product} %>').modal('show');

products_controller_decorator..rb

# This file already exists, you'll just append to it...

Spree::ProductsController.class_eval do

  respond_to :js, :html
  
  # custom method/endpoint that our ajax call will hit. This will respond to the ajax call and render
  # a template in spree/products/add_to_cart.js.erb and you're @product varaible will be available
  def add_to_cart
    @product = Spree::Product.find(params[:id])
    # No need for this. 
    # render partial: "spree/products/cart_form", locals: {product: @product}
  end

end

routes.rb

# This adds a method to products called "add_to_cart" to the member (individual resource). If you run 
# rake routes in your console you'll see it requires an id #=> /products/:id/add_to_cart

resources :products, only: [] do
  get :add_to_cart, on: :member
end

show.haml

-# Let's use a link tag and utilize Rails's Ujs AJAX mechanism. The remote: true will make this into an ajax
-# call to the server with the application/js data type. (see the controller below)
= link_to "BUY", add_to_cart_product_path(product.id), remote: true, class: "btn btn-secondary pull-left"