Partials views and instance variables go nil

Hi everyone!

Model:http://pastie.org/511290
Controller:http://pastie.org/511285
View new:http://pastie.org/511286
View _form:http://pastie.org/511284

When I hit submit button there's an error for usuarios(users in
portuguese) instance. It does not seems to save usuarios even though it
seems to be posted by the form.
Error:http://pastie.org/511292

Well the error happens because a validation fails and so it rerenders
the form, however your create action doesn't setup @usuarios and so
the bit where you call collection_select fails.

If you look at your parameters although the id is being submitted,
it's not being submitted in the right place in your params hash:
you've got :usario => {:id => '6'}, whereas what you want is :venda =>
{:usario_id => '6', ... }

This is because of how you are using collection_select: you're telling
it that you are editing the id property of @usuario, but you're not:
you're editing the usario_id property of @venda. Fix that and it
should work (why not call collection_select on the form builder
object ?)

Fred