Spree add image to a product in controller

I'm working in a Rails Spree app, I was making my own form and methods to add products, everything was working fine until I had to add an image to my product.

In spree there is a assets table with a "viewable_id" field that contains the id of the product, but I have no idea of how, manually, add a new asset to this table from my ProductsController.

In my form I use a field file_field.

And this is the method in my ProductsController, it works fine but I can't realize how to add a new asset whit the id of my current product.

def newproduct

  @product = Product.new();

  #render '/spree/home/newproduct'

end

def createproduct

  @name = params[:product][:name];
  @description = params[:product][:description];
  @price = params[:product][:price];
  @image = params[:product][:image];
  @available_on = params[:product][:available_on];
  @meta_description = params[:product][:meta_description];
  @meta_keywords = params[:product][:meta_keywords];

  abort a

  @product = Product.new({
    :name => @name,
    :description => @description,
    :available_on => @available_on,
    :user_id => spree_current_user.id,
    :meta_description => @meta_description, 
    :meta_keywords => @meta_keywords,
    :price => @price,
    :image => @image});





  if @product.save!()
    redirect_to "/tutienda", :notice => "The product has been added";
  else
    render '/spree/products/newproduct'
  end

end