Trying to fill in a couple of text fields based on user selection from a drop-down list

I'm *very* new working with Ajax. In a data submission form, I'd like
some data to be filled in automatically (from the database) if a user
selects certain options from a drop-down list.

I have an Event model, and an Event submission form. If certain venues
are selected on the form, I'd like to pre-populate the phone number
and URL. So, if someone selects "Main Library" from the drop-down
list, the library phone number and URL would up on the form. (The drop-
down list is populated from my Venue model data).

Here's what I've tried without success. Any suggestions would be
greatly appreciated.

My submission view includes

      <div class="form_row">
      <label for="venue_id">Place*:</label> <%=
collection_select(:event, :venue_id, @venues_all, :id, :name, {:prompt
=> " Please select a location"}) %>
    </div>

        <%= observe_field("venue_id",
                          :frequency => 1,
                          :url => { :controller => 'event', :action =>
"venue_lookup" },
                          :with => "'id=' + value") %>

<div class="form_row">
  <label for="forinfo">Phone: </label><%= f.text_field :forinfo, :size
=> 50, :value => @user_phone %>
</div>

<div class="form_row">
<label for="url">Web site:</label> <%= f.text_field :url, :size =>
50, :value => @user_url %>
</div>

And then my event controller includes

  def venue_lookup
    @venue = Venue.find(params[:id])
    render :update do |page|
      page[:forinfo].value = @venue.contactinfo #this is in the
database for venues in the drop-down list
      page[:url].value = @venue.url #this is in the database for
venues in the drop-down list
    end
  end

I've got mixed model input on the form, so I also tried

  def venue_lookup
    @venue = Venue.find(params[:id])
    render :update do |page|
      page[:event_forinfo].value = @venue.contactinfo #this is in the
database for venues in the drop-down list
      page[:event_url].value = @venue.url #this is in the database for
venues in the drop-down list
    end
  end

but that's not working, and I can't figure out why.

Thanks.

Sharon