One button within loop (mass_assignt) - ruby-on-rails-3

Question

I have a form, which has loop as you can see below.
My aim:
text | checkbox |
text2 | checkbox |
text3 | checkbox |
text4 | checkbox |
______
button
------
If I clicked on button my table changes if i choose checkbox of course. So its One button in one form which has one loop.
PROBLEM:
Only last one is pasted successfully, (dont know why) e.g if i changed last item, this is ok, but when i changed first one, second one for example and last one, i get changes only in last one :(
DOESNT SEE MY LOOP i think :(
my show view
<%= form_for :task, :url => { :controller => :tasks, :action => :update } do |f| %>
<% #tasks.each_with_index do |task, index| %>
<%= f.fields_for task do |task_form| %>
<%= task_form.text_field :task %> | <%= task_form.check_box :is_active %> <%= task_form.hidden_field :id %>
<% end %>
<% end %>
<br/><br/>
<%= f.submit 'Update' %>
<% end %>
my controller:
def show
#tasks=Task.find(:all)
end
def update
tasks=Task.find(:all, :conditions => ["id=?", params[:task][:id]])
tasks.each do |task|
task.task=params[:task][:task]
task.save
end
end
And thats error i get:
{"utf8"=>"✓",
"authenticity_token"=>"vn0DUQhRa2zBguCFRf/fY/cYnzKYAkio0MwDUd4/mC8=",
"task"=>{"task"=>{"task"=>"dlaj testa!",
"is_active"=>"1",
"id"=>"11"}},
"commit"=>"Update"}
undefined method `each' for nil:NilClass

Answer 1

I believe it is saying that tasks.each do |task| is the cause. tasks is nil, which you define above.
Instead of
tasks=Task.find(:all, :conditions => ["id=?", params[:task][:id]])
I would probably write it as:
tasks = Task.where(id: params[:task][:id])
That will return an array of all of the tasks, even if it's an empty array.
Then calling .each should not fail, since it's empty.