rails 3 option_from_collection_for_select with selected null value

I'm experiecing some problems with option_from_collection_for_select when the selected value is nil.

I have User model:

class User < ActiveRecord::Base
  attr_accessible :email, :password, :password_confirmation, :remember_me, :username, :locale
end

and a Reservation one:

class Reservation < ActiveRecord::Base
  belongs_to :user
end

In the reservation add/edit page there is a select with the user is goiing to be associated with the reservation:

<%= select_tag('user_id', 
  options_from_collection_for_select(@users, :id, :username, @reservation.user.id)) %>

This works in the edit page when @reservation.user.id is not nil but I get the nil error in the add page. Just for now I check if @reservation.user.id is nil and I put the selected value depending if is nil or not; this works but I don't like this solution.

How can I solve this?

Solution

There are probably many ways around this:

At the very least, you'd have to put a conditional for the last argument for ofcfs.

options_from_collection_for_select(
  @users, :id, :username, 
  @reservation.user.blank? ? 0 :  @reservation.user.id)

Then, in your create action, you'd have to prepend an appropriate hash:

@users = User.all
@users.insert(0, {:id=>0,:username=>'PLEASE SELECT A USER'})

Then you'd have to validate that the value returned from the select isn't 0 before you save a reservation.

There are at least three or four other things I can think of that work around this