Rails - Apr 2006 - Many To Many''s Join table attributes + migrations

Hi all,

just joined and have only just kicked off with Rails (coming, but not  
departing, from WebObjects development). And so far there seems to be  
a lot of promise except for one thing I can''t figure out as yet --  
the above problem.

Say I have the following conceptual relationships:

	ITEMS <<--->> CARTS

i.e., each item can belong to one or more categories. So really we have
	
	ITEMS <<--> ITEMS_CARTS <-->> CARTS

where the join table, items_carts, has the dual primary keys  
{items_id, carts_id}.

However, in reality people can and will buy more than one of each  
item and so items_carts needs an additional attribute: quantity.

My question is how might the above be defined as a migration? In  
WebObjects when modelling the above entities (akin to migrations) you  
simply set the outer entities to propagate their primary keys.

I''ve searched google for quite some time but seem to keep finding  
either objections to allowing for multiple primary keys (even though  
many-to-many''s are common-place) or mention of work-a-rounds in  
progress.

Any ideas?
I''ve seen the following article but it doesn''t go into enough
detail
and straight away bypasses the abstract migration tool using instead  
database specific sql (which suggests that the built in features may  
not exist for this yet): http://jrhicks.net/Projects/rails/ 
has_many_and_belongs_to_many.pdf

On a similar note:
How does rails handle the plural expansion of words like
''category''.
i.e., if you have a migration called categories is it going to  
correctly use the singular for the model? That kind of naming scheme  
doesn''t seem to make sense in enough situations, but anyway...

Thanks in advance...

with regards,
--

Lachlan Deck

On Apr 10, 2006, at 6:16 AM, Lachlan Deck wrote:
> just joined and have only just kicked off with Rails (coming, but  
> not departing, from WebObjects development). And so far there seems  
> to be a lot of promise except for one thing I can''t figure out as
> yet -- the above problem.
>
> Say I have the following conceptual relationships:
>
> 	ITEMS <<--->> CARTS
>
> i.e., each item can belong to one or more categories. So really we  
> have
> 	
> 	ITEMS <<--> ITEMS_CARTS <-->> CARTS
>
> where the join table, items_carts, has the dual primary keys  
> {items_id, carts_id}.
The Rails default naming for HABTM tables is alphabetical order, so  
that table
should be:

	CARTS <<--> CARTS_ITEMS <-->> ITEMS

-- 
-- Tom Mornini

Hi there,

On 11/04/2006, at 1:16 AM, Tom Mornini wrote:
> On Apr 10, 2006, at 6:16 AM, Lachlan Deck wrote:
>
>> just joined and have only just kicked off with Rails (coming, but  
>> not departing, from WebObjects development). And so far there  
>> seems to be a lot of promise except for one thing I can''t
figure
>> out as yet -- the above problem.
>>
>> Say I have the following conceptual relationships:
>>
>> 	ITEMS <<--->> CARTS
>>
>> i.e., each item can belong to one or more categories. So really we  
>> have
>> 	
>> 	ITEMS <<--> ITEMS_CARTS <-->> CARTS
>>
>> where the join table, items_carts, has the dual primary keys  
>> {items_id, carts_id}.
>
> The Rails default naming for HABTM tables is alphabetical order, so  
> that table
> should be:
>
> 	CARTS <<--> CARTS_ITEMS <-->> ITEMS
No problems (just a typo) but my real question is how do you define a  
migration for CARTS_ITEMS so that it has the following attributes?
- carts_id (primary key)
- items_id (primary key)
- quantity

with regards,
--

Lachlan Deck

00X_create_carts_items.rb

class CreateCartsItems < ActiveRecord::Migration
  def self.up
    create_table :carts_items, :id => false do |t|
      t.column :cart_id, :integer
      t.column :item_id_id, :integer
      t.column :quantity, :integer
    end

    # there is no way to specify multiple primary keys
    execute "ALTER TABLE carts_items ADD PRIMARY KEY (cart_id,
item_id)"

    # alternative
    # add_index :carts_items, [:cart_id, item_id], :unique => true
  end

  def self.down
    # drop_index :cart_items, :cart_id # use if using alternative to primary
keys
    drop_table :carts_items
  end
end


On 4/10/06, Lachlan Deck <lachlan.deck@gmail.com>
wrote:
>
> Hi there,
>
> On 11/04/2006, at 1:16 AM, Tom Mornini wrote:
>
> > On Apr 10, 2006, at 6:16 AM, Lachlan Deck wrote:
> >
> >> just joined and have only just kicked off with Rails (coming, but
> >> not departing, from WebObjects development). And so far there
> >> seems to be a lot of promise except for one thing I can''t
figure
> >> out as yet -- the above problem.
> >>
> >> Say I have the following conceptual relationships:
> >>
> >>      ITEMS <<--->> CARTS
> >>
> >> i.e., each item can belong to one or more categories. So really we
> >> have
> >>
> >>      ITEMS <<--> ITEMS_CARTS <-->> CARTS
> >>
> >> where the join table, items_carts, has the dual primary keys
> >> {items_id, carts_id}.
> >
> > The Rails default naming for HABTM tables is alphabetical order, so
> > that table
> > should be:
> >
> >       CARTS <<--> CARTS_ITEMS <-->> ITEMS
>
> No problems (just a typo) but my real question is how do you define a
> migration for CARTS_ITEMS so that it has the following attributes?
> - carts_id (primary key)
> - items_id (primary key)
> - quantity
>
> with regards,
> --
>
> Lachlan Deck
>
>
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>
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Hi there,

On 11/04/2006, at 4:12 AM, Chris Hall wrote:
> 00X_create_carts_items.rb
>
> class CreateCartsItems < ActiveRecord::Migration
>   def self.up
>     create_table :carts_items, :id => false do |t|
>       t.column :cart_id, :integer
>       t.column :item_id_id, :integer
>       t.column :quantity, :integer
>     end
>
>     # there is no way to specify multiple primary keys
>     execute "ALTER TABLE carts_items ADD PRIMARY KEY (cart_id,  
> item_id)"
>
>     # alternative
>     # add_index :carts_items, [:cart_id, item_id], :unique => true
>   end
>
>   def self.down
>     # drop_index :cart_items, :cart_id # use if using alternative  
> to primary
> keys
>     drop_table :carts_items
>   end
> end
Thanks Chris.
>> No problems (just a typo) but my real question is how do you define a
>> migration for CARTS_ITEMS so that it has the following attributes?
>> - carts_id (primary key)
>> - items_id (primary key)
>> - quantity
>>
>> with regards,
>> --
>>
>> Lachlan Deck
with regards,
--

Lachlan Deck