Ajax button in rails - Unprocessable Entity - ruby-on-rails

Question

I created an ajax button to create a transaction, but when I click on it, the Transaction.count doesn't increment.
I have three models : User, Product and Transaction
User and Product have a many to many association through transaction, that belongs to both.
Here is the create method in my app/controllers/transactions_controller.rb
def create
#product = Product.find(params[:transaction][:product_id])
#transaction = Transaction.new(transaction_params)
#transaction.save!
respond_to do |format|
format.html { redirect_to #product }
format.js
end
end
def transaction_params
params.require(:transaction).permit(:product_id, :start_date, :end_date)
end
Here is my button view app/controllers/transactions/buttons/_wants.html.erb
<div class="wants_button product_<%=product.id%>">
<% if current_user %>
<%= form_for(current_user.transactions.build(product_id: product.id),
remote: true) do |f| %>
<div><%= f.hidden_field :product_id %></div>
<%= f.submit t('button.ownership.create'), class: "btn btn-success btn-small" %>
<% end %>
<% end %>
</div>
And here my app/views/transactions/create.js.erb
$(".wants_button.product_<%=#product.id%>").html("<%= escape_javascript(render('transactions/ownership/buttons/wants_not', product: #product)) %>")
At least, what returns the server :
Started POST "/transactions" for 127.0.0.1 at 2013-07-22 16:05:52 +0200
Processing by TransactionsController#create as JS
Parameters: {"utf8"=>"✓", "transaction"=>{"product_id"=>"239"}, "commit"=>"Acquisition"}
User Load (0.3ms) SELECT "users".* FROM "users" WHERE "users"."remember_token" = 'A0J7Ii29W7NPO0ECCccm6g' LIMIT 1
Product Load (0.2ms) SELECT "products".* FROM "products" WHERE "products"."id" = ? ORDER BY products.created_at DESC LIMIT 1 [["id", "239"]]
(0.1ms) begin transaction
(0.1ms) rollback transaction
Completed 422 Unprocessable Entity in 12ms
Do you know what could interfere with the creation of a transaction ?

Answer 1

Ran into a similar problem. Make sure you are sending the CSRF authenticity token - using rails' form_tag or what have you automatically includes it for you, but manual AJAX request do not.