I have a keychain object. keychain has_many credentials.

I'm trying to write the view code to add a new credential to a keychain. This is the code I have:

<% form_for(@keychain) do |f| %>
          <tr>
            <td><%= f.select "credentials[]", current_account.services.collect{ |s| 
                    [s.friendly_name, s.id] } %></td>
            <td><%= f.text_field 'credentials', :username %></td>
            <td><%= f.password_field 'credentials', :password %></td>
          </tr>
<% end %>

But it fails with this message:

NoMethodError in Keychains#new

Showing app/views/keychains/_keychain_form.html.erb where line #32 raised:

undefined method `credentials[]' for #

What am I doing wrong?