Hidden field in rails form

You are missing on = after <%. The equal sign is needed whenever you want to the result appears on the HTML, so it is used with the field tags methods or render, for instance. You should not use the equal when using a if, for example, because this is not what you want to print (well, it can be, but most likely it isn't)

<%= form_for :taskid, :url => {:action=>"index", :controller=>"statistics"}, :html => {:class => "nifty_form", :method => "GET"} do |f| %>  <%= f.hidden_field :task_id, :value => task.id%>  <td><%= f.submit "اختر مهمة لاظهار احصائياتها منفرده"%></td><% end %>

However, as @AntonGrigoriev pointed out, you should use a object if you have, like this

<%= form_for @task, :url => {:action=>"index", :controller=>"statistics"}, :html => {:class => "nifty_form", :method => "GET"} do |f| %>

or you can simply use the hidden_field_tag

<%= hidden_field_tag :task_id, task.id %>

Hi please test with following code to send hidden value in rails, I have tried and worked for one of my application :

hidden_field_tag(name, value = nil, options = {}) publiceg:

<%= hidden_field_tag(:field_name,value=@offer_status)%>