Rails select has_many though get the right Id - ruby-on-rails

Question

I'm trying to pass the size of a product to a order with select. the product has many sizes with has_many though.
the size.id on the orger page did not match with the size.id that i choose on the product page.
someone know why?
for exemple if choose product size.id(2) on the order page show size.id(4)
Size Model
attr_accessible :size
has_many :orders
has_many :sizeship
has_many :products, :through => :sizeship
Product Model
has_many :orders
has_many :sizeships
has_many :sizes, through: :sizeships
Order Model
belongs_to :cart
belongs_to :product, touch: true
belongs_to :size
Cart Model
belongs_to :user
has_many :orders
Product Controller
def show
#product = Product.cached_find(params[:id])
#sizes_for_dropdown = #product.sizes.collect { |s| [s.size, s.id] }
end
Cart Controller
def add
product = Product.find(params[:id])
if product
if product.buyable?(current_user)
current_user.cart = Cart.new #if current_user.cart.nil?
size = product.sizes.find_by_id(params[:size_id])
quantity = params[:quantity].to_i > 0 ? params[:quantity].to_i : 1
order = current_user.cart.orders.find_by_product_id_and_status_and_size_id(product.id, nil, product.sizes.nil? ? nil : product.sizes.find { |l| l.id } )
if order.nil? # create new order
order = Order.new
order.product = product
order.size = product.sizes.find { |k| k.id }
current_user.cart.orders << order
else # increase quantity
order.quantity += quantity
order.save
end
redirect_to product_path(product)
flash[:success] = "Success"
else
redirect_to product_path(product)
flash[:error] = " Error"
end
else
redirect_to :shop, flash[:error] = 'Error'
end
end
Log
Started POST "/carts/add/yy" for 127.0.0.1 at 2016-11-17 00:11:16 -0200
Processing by CartsController#add as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"Hf7eLJPojw+u/HCGuQtAVzADC7v0jpkw5mJ0sCmx0u4=", "score"=>"", "quantity"=>"1", "size_id"=>"1", "commit"=>"add to basket", "id"=>"yy"}
Redirected to http://localhost:3000/products/yy
Completed 302 Found in 3471.4ms