Partials views and instance variables go nil

Hi everyone!

View new:
View _form:

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.

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 ?)