custom.js

$(".completed").live("change", function(task) {
    $.ajax({
      url: "/admin/tasks/completed",
      beforeSend: function() { alert("Hi"+task) },
      data: "id="+task, 
      success: function() { alert('Bye') }
    });
  });

index.html.erb

<% @tasks_responsible.each do |t| %>
            <tr>
              <td>
                <%= check_box_tag "completed", t.id, false, :class => "completed" %>
              </td>
              <td><%= t.task_type %></td>
              <td><%= link_to "#{truncate(t.subject, :length => 20)}", admin_task_path(t) %></td>
              <td><%= t.responsible(&:name) %></td>
              <td><%= link_to t.client, admin_client_path(t.client) unless t.client.nil? %></td>
              <td><%= t.deadline.strftime("%d/%m/%y %H:%M") %></td>
              <td><%= t.author(&:name) %></td>
              <td>
                <a data-toggle="modal" href="#editTask<%= t.id %>"><%= image_tag "edit.png" %></a>
                <a data-toggle="modal" href="#deleteTask<%= t.id %>"><%= image_tag "delete.png" %></a>
              </td>
            </tr>
          <% end %>

tasks_controller.rb

def completed
    @task = Task.find(params[:id])

    if @task.completed
      @task.update_attributes :completed, true
    else 
      @task.update_attributes :completed, false
    end
  end

terminal

Started GET "/admin/tasks/completed?id=[object%20Object]" for 127.0.0.1 at 2012-04-26 18:27:48 +0200
Processing by Admin::TasksController#show as */*
  Parameters: {"id"=>"completed"}
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
  Task Load (0.1ms)  SELECT "tasks".* FROM "tasks" WHERE "tasks"."id" = ? LIMIT 1  [["id", "completed"]]
Completed 500 Internal Server Error in 3ms

ActiveRecord::RecordNotFound (Couldn't find Task with id=completed):
  app/controllers/admin/tasks_controller.rb:15:in `show'


  Rendered /Users/znowm4n/.rvm/rubies/ruby-1.9.3-p0/lib/ruby/gems/1.9.1/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/_trace.erb (2.3ms)
  Rendered /Users/znowm4n/.rvm/rubies/ruby-1.9.3-p0/lib/ruby/gems/1.9.1/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (0.7ms)
  Rendered /Users/znowm4n/.rvm/rubies/ruby-1.9.3-p0/lib/ruby/gems/1.9.1/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/diagnostics.erb within rescues/layout (9.3ms)