Rails - May 2006 - How can I set up relations for these models?

Hello!

I have a legacy DB and want to use ActiveRecord for it.
However, I can''t set up a relation correctly.

Here''s table spec:

[products]
id
name
price

[subproducts]
product_id
subproduct_id


Some example data:

[products]
1, "Computer", 500.00
2, "Monitor", 200.00
3, "Printer", 100.00
4, "System", 700.00
5, "System2", 650.00

"System" is made up of "Computer", "Monitor", and
"Printer"
"System2" is made up of "Computer" and "Printer"

So [subproducts] table will have the following rows.

4, 1
4, 2
4, 3
5, 1
5, 2

"Computer" can be sold by itself or as a part of "System".

class Product < ActiveRecord::Base
  #???
end

How do I set up relations?
It seems like a many-to-many relation.
But I''m stuck here.

Thansk.
Sam




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First thing I would do is try this:

class Product < ActiveRecord::Base
   has_many :subproducts
end

class Subproduct < ActiveRecord::Base
  belongs_to :product
  set_primary_key "subproduct_id"
end


Now I don''t know what subproducts are but I assume that
''product_id''
is the parent id and subproduct_id is the primary key of the table but
a foreign key to the actual product itself.

That gets tricky but not really impossible.




p = Product.find(1)
new_product = Product.create :name=>"Computer", :price=>500
p.subproducts << SubProduct.create :subproduct_id =>new_product.id


And you could pretty that up by making your own add_subproduct method
on your model. A rushed and untested example might be this:

class Product < ActiveRecord::Base
   has_many :subproducts


   def add_subproduct(product_id)
    self.subproducts << Subproduct.create :subproduct_id => product_id
   end

   def remove_subproduct(product_id)
      self.subproducts.remove(product_id)
   end
end


p = Product.find(1)
s = Product.find(2)
p.add_subproduct s.id
p.remove_subproduct s.id

Would that work?



On 5/19/06, Sam Kong <sam.s.kong@gmail.com> wrote:
> Hello!
>
> I have a legacy DB and want to use ActiveRecord for it.
> However, I can''t set up a relation correctly.
>
> Here''s table spec:
>
> [products]
> id
> name
> price
>
> [subproducts]
> product_id
> subproduct_id
>
>
> Some example data:
>
> [products]
> 1, "Computer", 500.00
> 2, "Monitor", 200.00
> 3, "Printer", 100.00
> 4, "System", 700.00
> 5, "System2", 650.00
>
> "System" is made up of "Computer", "Monitor",
and "Printer"
> "System2" is made up of "Computer" and
"Printer"
>
> So [subproducts] table will have the following rows.
>
> 4, 1
> 4, 2
> 4, 3
> 5, 1
> 5, 2
>
> "Computer" can be sold by itself or as a part of
"System".
>
> class Product < ActiveRecord::Base
>   #???
> end
>
> How do I set up relations?
> It seems like a many-to-many relation.
> But I''m stuck here.
>
> Thansk.
> Sam
>
>
>
>
> --
> Posted via http://www.ruby-forum.com/.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

Hi, Brian,
Thank you for the answer.

I need to make one thing clear.

subproducts table is just a join table which doesn''t have a primary 
key(id).
See the example data for subproducts.

product_id, subproduct_id
4, 1
4, 2
4, 3
5, 1
5, 2

subproduct_id can''t be a primary_key just by itself.
Primary key would be the composite of product_id and subproduct_id.

This is a many-to-many relation.
But the difference is that this relation is between one table instead of 
two.

Usually a many-to-many is like between student-to-class.
But my case is like product-to-product.

What do you think?

Sam

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

Do you mean to say that it is possible for a subproduct to belong to
more than one product? Cos that''s what this implies.


That''s easy though... you should look here:

http://rails.techno-weenie.net/tip/2006/4/12/self_referential_many_to_many_relationships

That will do it for you then!


On 5/19/06, Sam Kong <sam.s.kong@gmail.com> wrote:
> Hi, Brian,
> Thank you for the answer.
>
> I need to make one thing clear.
>
> subproducts table is just a join table which doesn''t have a
primary
> key(id).
> See the example data for subproducts.
>
> product_id, subproduct_id
> 4, 1
> 4, 2
> 4, 3
> 5, 1
> 5, 2
>
> subproduct_id can''t be a primary_key just by itself.
> Primary key would be the composite of product_id and subproduct_id.
>
> This is a many-to-many relation.
> But the difference is that this relation is between one table instead of
> two.
>
> Usually a many-to-many is like between student-to-class.
> But my case is like product-to-product.
>
> What do you think?
>
> Sam
>
> --
> Posted via http://www.ruby-forum.com/.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

Oh, I think that it''s exactly what I want.
Thanks!

Sam

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

Sam Kong wrote:
> Oh, I think that it''s exactly what I want.
> Thanks!
That should work fine if all you need is a join table.  If you want to 
keep additional attributes in the subproducts table (like maybe number 
of units?), you''ll want to use a self-referential join model.

http://blog.hasmanythrough.com/articles/2006/04/21/self-referential-through

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

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

Josh Susser wrote:
> Sam Kong wrote:
>> Oh, I think that it''s exactly what I want.
>> Thanks!
> 
> That should work fine if all you need is a join table.  If you want to 
> keep additional attributes in the subproducts table (like maybe number 
> of units?), you''ll want to use a self-referential join model.
> 
> http://blog.hasmanythrough.com/articles/2006/04/21/self-referential-through
Wow, how did you know that?
I need additional attributes in the subproducts table.
I was thinking about it and you already answered.

Thank you so much.
Sam

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

Sam Kong wrote:
> Wow, how did you know that?
> I need additional attributes in the subproducts table.
> I was thinking about it and you already answered.
I''ve been answering questions on this list for a while.  I guess
I''m
learning what usually comes next :-)

cheers!

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

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