How to make chainable methods (scopes) with conditions not tied to database

I have a model Item, which has a relation to itself.

class Item < ActiveRecord::Base
  has_many :subitems, :class_name => "Item", :foreign_key => "superitem_id"
  belongs_to :superitem, :class_name => "Item"
end

And I want to query all items which have a parent. Firstly I've tried to check if parent_id is present Item.where("superitem_id != ?", false), or something like this. But it doesn't work. Although that item has superitem_id, superitem can be already destroyed. So I have to do it with class m开发者_StackOverflow社区ethod

def self.with_superitems
  items = []
  self.find_each do |i|
    items << i if i.superitem
  end
  return items
end

But it makes chaining impossible, and I want to chain it with similar methods, like

def self.can_be_stored
  items = []
  self.find_each do |i|
    items << i if i.can_be_stored?
  end
  return items
end

Is it possible to achieve the same results with scopes? Or what would you do?

I've had a similar issue in the past. It's sometimes difficult to get round it. I found a hack-ish way of doing it for my purposes so hope this will help...

 ids = []
 self.find_each do |i|
    ids << i.id if i.superitem
 end
Model.where('id in (?)', ids)

In rails 2 i would have done this

items = Item.find(:all, :include => [:superitems], :conditions => ["superitems.id is not null"])

the rails3 equivalent of this is

Item.includes([:superitem]).where("superitems.id is not null").all

This way you're pulling in the parent and testing if the id field on the superitem side of the join has an id. if it doesn't, it's because there's no superitem there (or, technically, it could be there but have no id. But this would normally never happen).

The following will get all the items with a parent, I'm not sure what you mean when you say "Although that item has superitem_id, superitem can be already destroyed"

items = Item.where("superitem_id IS NOT NULL")