Ryan Bates' nifty_scaffolding, for example, does this

edit.html.erb

<%= render :partial => 'form' %>

new.html.erb

<%= render :partial => 'form' %>

_form.html.erb

<%= form_for @some_object_defined_in_action %>

That hidden state makes me feel uncomfortable, so I usually like to do this

edit.html.erb

<%= render :partial => 'form', :locals => { :object => @my_object } %>

_form.html.erb

<%= form_for object %>

So which is better: a) having partials access instance variables or b) passing a partial all the variables it needs?

I've been opting for b) as of late, but I did run into a little pickle:

some_action.html.erb

<% @dad.sons.each do |a_son| %>
<%= render :partial => 'partial', :locals => { :son => a_son } %>
<% end %>

_partial.html.erb

The son's name is <%= son.name %>
The dad's name is <%= son.dad.name %>

son.dad makes a database call to fetch the dad! So I would either have to access @dad, which would be going back to a) having partials access instance variables or I would have to pass @dad in locals, changing render :partial to <%= render :partial => 'partial', :locals => { :dad => @dad, :son => a_son } %>, and for some reason passing a bunch of vars to my partial makes me feel uncomfortable. Maybe others feel this way as well.

Hopefully that made some sense. Looking for some insight into this whole thing... Thanks!