html - Add instruction to drop down select box in Rails? -

html - Add instruction to drop down select box in Rails? -

i displaying list of users (from users model) in rails 3 application. in helper (in case it's faults) have following:

def user_all_select_options user.all.map{ |user| [user.name, user.id] } end

in view have following:

<%= f.select :user_id, user_all_select_options %>

this creates drop downwards list first users name pre-selected @ top of list. trying add together instruction(?) @ top of list unselectable. thought prevent top user getting accidentally set required user when don't want be.

so rather select box looking this:

i this:

is possible, if so, how??

pass :prompt option.

<%= f.select :user_id, user_all_select_options, :prompt => 'select user' %>

html ruby-on-rails-3 select helper