Rails - Jul 2006 - Polymorphic Association with Single Table Inheritance?

Hello,

is it possible to setup a model/table schema like this:

Groupable  --> Membership <-- Group
    ^                           ^
    |                           |
   User                      UserGroup


I tried the following but failed:

Groupable (table with ''type'' column)
has_many :memberships, :as => :groupable
has_many :groups, :through => :memberships

Membership
belongs_to :groupable, :polymorphic => true
belongs_to :group, :polymorphic => true

Group (table with ''type'' column)
has_many :memberships, :as => :group
has_many :groupables, :through => :memberships


And the inherited tables:

User < Groupable
has_many :user_groups, :through => :memberships

UserGroup < Group
has_many  :users, :through => :memberships


Setting up an user, a membership and an user_group
works, but it throws an exception on accessing
user_group.users:
>> ruby script/console
>> u = User.create
>> m = Membership.create
>> ug = UserGroup.create
>> u.memberships << m
>> ug.memberships << m
>> ug.users
ActiveRecord::HasManyThroughAssociationPolymorphicError: 
ActiveRecord::HasManyThroughAssociationPolymorphicError


Thanks for any help!

Greets,
Stephan

-- 
Posted via http://www.ruby-forum.com/.

Stephan Schubert wrote:
> Hello,
> 
> is it possible to setup a model/table schema like this:
> 
> Groupable  --> Membership <-- Group
>     ^                           ^
>     |                           |
>    User                      UserGroup
> 
> 
> I tried the following but failed:
> 
> Groupable (table with ''type'' column)
> has_many :memberships, :as => :groupable
> has_many :groups, :through => :memberships
> 
> Membership
> belongs_to :groupable, :polymorphic => true
> belongs_to :group, :polymorphic => true
> 
> Group (table with ''type'' column)
> has_many :memberships, :as => :group
> has_many :groupables, :through => :memberships
> 
> 
> And the inherited tables:
> 
> User < Groupable
> has_many :user_groups, :through => :memberships
> 
> UserGroup < Group
> has_many  :users, :through => :memberships
> 
> 
> Setting up an user, a membership and an user_group
> works, but it throws an exception on accessing
> user_group.users:
> 
>>> ruby script/console
>>> u = User.create
>>> m = Membership.create
>>> ug = UserGroup.create
>>> u.memberships << m
>>> ug.memberships << m
>>> ug.users
> ActiveRecord::HasManyThroughAssociationPolymorphicError: 
> ActiveRecord::HasManyThroughAssociationPolymorphicError
has_many :through works just fine with STI, however it has some issues 
with polymorphism.  I have an explanation and a partial workaround here:
http://blog.hasmanythrough.com/articles/2006/04/03/polymorphic-through

But it doesn''t look like you are doing STI at all, just garden-variety 
polymorphism. The terminology can be confusing, as generic OOP 
inheritance allows polymorphism through specialization, meaning a 
subclass can be used in place of a superclass. But in Rails STI just 
means the models get their data from the same table, which allows the 
type information to be encapsulated in the model itself (in the
''type''
field in the table), rather than being placed alongside the groupable_id 
foreign key in the groupable_type field. You only need to have the type 
in one place, so only use STI or :as => :typeable.

--
Josh Susser
http://blog.hasmanythrough.com

-- 
Posted via http://www.ruby-forum.com/.

Josh Susser wrote:
> But it doesn''t look like you are doing STI at all, just
garden-variety
> polymorphism. 
Ah, got the point now. A table containing all attributes of all possible
''subclasses''..

Thank you, read all your blog entries about rails - pretty good stuff :)

Greets,
Stephan

-- 
Posted via http://www.ruby-forum.com/.

Hi Steven,

I was reading your post to the ruby on rails mailing list regarding  
single table inheritance, polymorphism and groups. (http:// 
lists.rubyonrails.org/pipermail/rails/2006-July/056582.html)

Did you ever get this sorted out?

Cheers Leevi
	
Leevi Graham
Front End Developer - User Interface Designer
  www.leevigraham.com |  Skype Me: leevi_graham
  MSN Messenger: info@leevigraham.com
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