Conditional form input field types - ruby-on-rails

Question

Rails 2.3.11
I would like to use a text_field to input data under one condition, otherwise using a selection box. Right now, my code looks like this:
views/posters/new.html.erb
<% form_for #poster, :html => {:multipart => true} do |f| %>
<%= f.error_messages %>
<p>
<%= f.label :image %> - We're not going to enlarge it for you, so please upload the biggest copy you can!<br />
<%= f.file_field :image %><br />
</p>
<p>
<% if current_user.admin? && params[:event_id] && !current_user.events.find_by_id(params[:event_id]) && Event.find_by_id(params[:event_id]) %>
<%= f.label "Event ID" %><br />
<%= f.text_field :event_id, :value => params[:event_id] %>
<% else %>
<%= f.label :event_id %><br />
<%= f.select :event_id, #events, :selected => params[:event_id].to_i %>
<% end %>
</p>
<p>
<%= f.submit 'Create' %>
</p>
<% end %>
controllers/posters_controller.rb
def new
#poster = Poster.new
current_user ||= User.find_by_id session[:user_id]
#events = [["Don't attach to an event", '']]
current_user.events.each {|event| #events << [event.title, event.id]}
respond_to do |format|
format.html # new.html.erb
format.xml { render :xml => #poster }
end
end
Error message: http://cl1p.net/halp
How can I use two different types of form input fields for the same parameter (but each under a different condition, not simultaneously)?
Update: I think the problem stems from the issue of Rails putting the previously-submitted information back into their respective input fields. This explains why no tantrum is thrown when a file that passes all the validation tests (that is, a PNG less than 3 MB), but breaks down when nothing (or anything that doesn't meet that condition) is attached.

Answer 1

Firstly, I don't understand why you are manually setting the values on the text box and select box. Usually, Rails does this for you, but without the form definition, I can't tell if you really need to do this.
Even then, given the information, I think it's safe to say that what you need is this:
f.text_field :event_id
...
f.select :event_id, #events
This ought to work for what you intend to do. I'm not sure what it has to do with the submitting of a file, but yes you are right about the previously-submitted part. The unexpected nil stems from this:
params[:event_id].to_i
Unless you are setting this parameter entry manually inside your controller, you will not be able to cast it to an integer if it is nil. If you go with using the basic form helper calls this goes away.