Mel's solution

gistfile1.rb

# Mel,
# 
#   Sorry, I read too much of a previous problem I had into your question and
# probably spun you off in the wrong direction.  Jim's comment points you in
# the right direction, but its only going to get you so far because of a
# limitation in combining polymorphism and HasManyThrough.  You will run into
# this error trying to do what you want to do:
#   
#   http://rubydoc.info/github/rails/rails/master/ActiveRecord/HasManyThroughAssociationPolymorphicSourceError
#   
#   
# but given you probably don't want the :through par anyway (that would ignore
# the :quantity), I think we can do exactly what you need.
# 
# watch this code:

# lets create a small inventory:
apple = Item.create(:name => "Apple")
banana = Item.create(:name => "Banana")
strawberry = Item.create(:name => "Strawberry")
get_well_card = Item.create(:name => "Get Well Soon Card")


# lets build a fruit basket
fruit_basket = Kit.create(:name => "Fruit Basket")

# and put a bunch of fruit in it.
fruit_basket.kit_parts << KitPart.create(:part => strawberry, :quantity => 7)
fruit_basket.kit_parts << KitPart.create(:part => strawberry, :quantity => 7)
fruit_basket.kit_parts << KitPart.create(:part => banana, :quantity => 2)

# and we can see what kit parts it contains
fruit_basket.kit_parts
# => [#<KitPart id: 1, kit_id: 1, part_id: 3, part_type: "Item", quantity: 7>, #<KitPart id: 2, kit_id: 1, part_id: 2, part_type: "Item", quantity: 2>, #<KitPart id: 3, kit_id: 1, part_id: 1, part_type: "Item", quantity: 3>] 

# and we have a convenience method that unrolls that for us.  Note the implementation isn't a has many through because of the polymorphic error that causes.  Rails doesn't know how to join 2 disparate tables:
fruit_basket.parts
#  => [#<Item id: 3, name: "Strawberry">, #<Item id: 2, name: "Banana">, #<Item id: 1, name: "Apple">]


# lets create a get well package that includes a kit (our fruit basket) and an item (our get well card)
get_well_package = Kit.create(:name => "Get Well Basket")
get_well_package.kit_parts << KitPart.create(:part => fruit_basket, :quantity => 1)
get_well_package.kit_parts << KitPart.create(:part => get_well_card, :quantity => 1)

# and we can inspect it the same way as above:
get_well_package.kit_parts
# => [#<KitPart id: 4, kit_id: 2, part_id: 1, part_type: "Kit", quantity: 1>, #<KitPart id: 5, kit_id: 2, part_id: 4, part_type: "Item", quantity: 1>]
 
get_well_package.parts
#  => [#<Kit id: 1, name: "Fruit Basket">, #<Item id: 4, name: "Get Well Soon Card">]
  
  
  
# Here is the code that makes this happen:



class CreateKits < ActiveRecord::Migration
  def change
    create_table :kits do |t|
      t.string :name
    end
  end
end



class CreateItems < ActiveRecord::Migration
  def change
    create_table :items do |t|
      t.string :name
    end
  end
end



class CreateKitParts < ActiveRecord::Migration
  def change
    create_table :kit_parts do |t|
      t.integer :kit_id
      t.integer :part_id
      t.string :part_type
      t.integer :quantity
    end
  end
end




class Item < ActiveRecord::Base
  has_many :kit_parts, :as => :part
end




class Kit < ActiveRecord::Base
  has_many :kit_parts
  has_many :containing_kits, :class_name => "KitParts", :as => :part
  
  # if you want to see an Exception that looks like the name was chosen
  # by a Java developer, uncomment this association, comment out the
  #'parts' method below, and try the sample code.
  #
  # has_many :parts, :through => :kit_parts
  #
  # rails can't go through a polymorphic relationship.
  
  def parts
    kit_parts.collect do |kp|
      kp.part
    end
  end
end


class KitPart < ActiveRecord::Base
  belongs_to :kit
  belongs_to :part, :polymorphic => true
end

This is why I love Ruby and RoR!
Although the problem might be complex to wrap your head around, the solution did not require any complex code...

Mel, I'm glad I saw your post to be able to look at this. I've struggled with this before and solved my situation with some fugly code.

@bokmann: thanks for your help with this problem. just a quick message to let you know i've found the code. i haven't studied it yet but, and especially after reading your comments on my gist, i think i can see where i went wrong: trying to maintain the traversal path from item to kit... i think...

Sorry to throw so many solutions at you - the last one best fits what you're trying to do. ActiveResource has some dead-end paths when using polymorphism or STI Glad I could help.

…

-db

On Mon, Feb 27, 2012 at 1:54 PM, Mel Riffe < ***@***.*** > wrote: @bokmann: thanks for your help with this problem. just a quick message to let you know i've found the code. i haven't studied it yet but, and especially after reading your comments on my gist, i think i can see where i went wrong: trying to maintain the traversal path from item to kit... i think... --- Reply to this email directly or view it on GitHub: https://gist.github.com/1915372