Help - undefined method `merge' for error.

I'm getting this error due to a change necessary for my form.

Initially - I was using formhelper where the form_for looked like this:
<%= form_tag :action => 'create_position' %>

Now I've switched to the scaffold_resource in edge rails where the
forms are setup as such:
<% form_for(:position, :url => positions_path) do |f| %>

So I know that this form_for will also accept the regular formhelpers.
However Im running into a problem with a field, where if i don't add
the f (i.e. f.select) the page loads fine but the input is never put
into the database.
If I put the f on it generates the undefined method merge error.
Here is the field:

<%= f.select 'pay', 'id', Pay.find(:all).collect {|p| [p.name, p.id]}%>

Hoping someone can help.
TIA
Stuart