User.rb (id,name)
    default_scope :conditions => 'users.deleted_at IS NULL'

NavItem.rb (id,user_id, friend_id)

items = NavItem.find_all_by_user_id(current_user.id)
items.each do |item|
    user = User.find(item.friend_id)
end

class User < ActiveRecord::Base
  attr_accessible :deleted_at, :name

  default_scope :conditions => 'users.deleted_at IS NULL'

  has_many :nav_items
end


class NavItem < ActiveRecord::Base
  attr_accessible :friend_id, :user_id

  belongs_to :user

  scope :without_deleted_users, where(:user_id => User.scoped)
end

NavItem.without_deleted_users
  NavItem Load (0.2ms)  SELECT "nav_items".* FROM "nav_items" WHERE "nav_items"."user_id" IN (SELECT "users"."id" FROM "users" WHERE (users.deleted_at IS NULL))