Rails - Aug 2009 - Active Record Associations and application outline advice

Hi if i have one model say with objects for each letter of the alphabet
e.g a,b,c ... x,y,z and i want to allow users to create there own
favourite pairings of these e.g.

ab, cd, ef, gp and allow them to discuss via comments on each pairing,
whats teh best way to model this in rails?

the order of teh letters DOESNT matter so ab IS the SAME as ba. Id
rather avoid duplication in my "Pairings" table because that would
mean
id have to assign user commments to both ab and ba. This would get out
of control especially if i then wanted to offer 3 letter combos e.g. abc
would also be saved as bca and cab.
This is actually my current setup but it doesnt sit well with me.

Can anyone suggest a better setup?

Sorry for the boring example of letters but it helps me write more
concisely. If you need more info id be more than happy to provide.
-- 
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Adam Akhtar wrote:
> Hi if i have one model say with objects for each letter of the alphabet
> e.g a,b,c ... x,y,z and i want to allow users to create there own
> favourite pairings of these e.g.
> 
> ab, cd, ef, gp and allow them to discuss via comments on each pairing,
> whats teh best way to model this in rails?
Use a Pairing class with a polymorphic association.
> 
> the order of teh letters DOESNT matter so ab IS the SAME as ba. Id
> rather avoid duplication in my "Pairings" table because that
would mean
> id have to assign user commments to both ab and ba.
Sort the object IDs in some unambiguous way before creating a pairing.


Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
Posted via http://www.ruby-forum.com/.

Hi Thanks for your reply and your suggestions. Im a bit confused, could 
you help clarify the following -
>> Hi if i have one model say with objects for each letter of the alphabet
>> e.g a,b,c ... x,y,z and i want to allow users to create there own
>> favourite pairings of these e.g.
>> 
>> ab, cd, ef, gp and allow them to discuss via comments on each pairing,
>> whats teh best way to model this in rails?
> 
> Use a Pairing class with a polymorphic association.
I thought polymorphic associations are used when you have several 
different models which have the same relationship with anohter one. I.e. 
You want to have comments associated with your models for Photos, Posts 
and Products for example. In this case however only my alphabet model 
has a relationship with the pairings model. So im not sure how i 
implement a polymorphic association here and how that that allow me to 
create only one pairing record to represent ab or ba.


my current setup is
class Pairing < ActiveRecord::base
belongs_to :alphabet
belongs_to :alphabetpartner, :class_name => "Alphabet",
:foreign_key =>
"alphabetpartner_id"

class Alphabet < ActiveRecord::Base
  has_many :pairings
  has_many :alphabetpartners,

but this requires me to create two records in hte pairings table for ab 
and ba.
>> the order of teh letters DOESNT matter so ab IS the SAME as ba. Id
>> rather avoid duplication in my "Pairings" table because that
would mean
>> id have to assign user commments to both ab and ba.
> 
> Sort the object IDs in some unambiguous way before creating a pairing.
Can you explain more about what you mean here and are you saying that i 
have to have two pairs in my pairing table for ab and ba?

-- 
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Adam Akhtar wrote:
[...]
[...]
>> Use a Pairing class with a polymorphic association.
> 
> I thought polymorphic associations are used when you have several 
> different models which have the same relationship with anohter one. I.e. 
> You want to have comments associated with your models for Photos, Posts 
> and Products for example. In this case however only my alphabet model 
> has a relationship with the pairings model. So im not sure how i 
> implement a polymorphic association here and how that that allow me to 
> create only one pairing record to represent ab or ba.
On rereading, I realize I was wrong.  I thought you had different 
*classes* A-Z, but I realize now that you don''t, so that you
don''t need
a polymorphic association after all.
> 
> 
> my current setup is
> class Pairing < ActiveRecord::base
> belongs_to :alphabet
> belongs_to :alphabetpartner, :class_name => "Alphabet",
:foreign_key =>
> "alphabetpartner_id"
That looks right.
> 
> class Alphabet < ActiveRecord::Base
>   has_many :pairings
>   has_many :alphabetpartners,
> 
> but this requires me to create two records in hte pairings table for ab 
> and ba.
No it doesn''t.

[...]
>> Sort the object IDs in some unambiguous way before creating a pairing.
> 
> Can you explain more about what you mean here and are you saying that i 
> have to have two pairs in my pairing table for ab and ba?
No, I am saying you would *not* have to have two pairs.  Rather, when 
you construct a Pairing, pick an unambiguous way of sorting the two 
Alphabet objects so that ab and ba will map to the same thing.  For 
example (untested):

class Pairing < AR::B
  before_create do
    [@alphabet, @alphabetpartner] = [@alphabet, 
@alphabetpartner].sort_by {|x| [x.name, x.id]}
  end
end

And BTW, @alphabetpartner should be @alphabet_partner.

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
Posted via http://www.ruby-forum.com/.

> No, I am saying you would *not* have to have two pairs.  Rather, when 
> you construct a Pairing, pick an unambiguous way of sorting the two 
> Alphabet objects so that ab and ba will map to the same thing.  For 
> example (untested):
> 
> class Pairing < AR::B
>   before_create do
>     [@alphabet, @alphabetpartner] = [@alphabet, 
> @alphabetpartner].sort_by {|x| [x.name, x.id]}
>   end
> end
Marnen many thanks again for your help.
So if i sort them by their name then id, and I have A and B with ids 1 
and 2 respectively then in my pairing controller id do
@pairing = Pairing.new(params[:pairing])
@pairing.save (Which would use the callback before_create above)

So A has an Ingredient_Partner B
i.e. A.alphabet_partners ===> B

But would it work the other way i.e.
B.alphabet_partners ==> A ?????

i thought it wouldnt without creating a 2nd pairing with the letters 
switched.


-- 
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"So A has an Ingredient_Partner B"

was a typo, should have read alphabet_partner
-- 
Posted via http://www.ruby-forum.com/.

Adam Akhtar wrote:
> 
> Marnen many thanks again for your help.
You''re welcome.
> So if i sort them by their name then id, and I have A and B with ids 1 
> and 2 respectively then in my pairing controller id do
> @pairing = Pairing.new(params[:pairing])
> @pairing.save (Which would use the callback before_create above)
> 
> So A has an Ingredient_Partner B
> i.e. A.alphabet_partners ===> B
That''s right.
> 
> But would it work the other way i.e.
> B.alphabet_partners ==> A ?????
> 
> i thought it wouldnt without creating a 2nd pairing with the letters 
> switched.
Look at the code I gave you, and think about how it would behave. 
Better yet, write some tests and try it!  Ask the computer, not me. :)

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
Posted via http://www.ruby-forum.com/.

Thanks again.
Ok i know why your saying to have a consistent sorting method, its so as 
to avoid a duplicate situation i.e. if i always sort by the letter and 
id then i will always save as ab and not ba. If theres already an ab 
there and i have some validations i can avoid reading in another ab. 
That bit i understand.

But the problem is i will only have, and ive tested this, a one way 
relationship. That is ''A'' is an "alphabet" which has
an
"alphabet_partner" of ''b''.

I.e. a.alphabet_partner = b.

But that''s the problem, ''B'' is just an
"alphabet_partner" in this
pairing table. There is nothing in the pairing table nor the 
associations currently set that say b
"B" is ALSO an "alphabet" which has an
"alphabet_partner" of ''A''.
i.e. b.alpahbet_partner == []. it will NOT yield ''A''

I have tried this with my setup and thats the result i get but also 
thinking about it makes me believe this can only be the end result.

I read up and i think (im not sure though) but i think i need something 
called bidirectional relationship. The only problem is I  dont know how 
to implement this.
Are you sure my results should be any different?
-- 
Posted via http://www.ruby-forum.com/.

Adam Akhtar wrote:
> Thanks again.
> Ok i know why your saying to have a consistent sorting method, its so as 
> to avoid a duplicate situation i.e. if i always sort by the letter and 
> id then i will always save as ab and not ba. If theres already an ab 
> there and i have some validations i can avoid reading in another ab. 
> That bit i understand.
OK.
> 
> But the problem is i will only have, and ive tested this, a one way 
> relationship. That is ''A'' is an "alphabet"
which has an
> "alphabet_partner" of ''b''.
> 
> I.e. a.alphabet_partner = b.
> 
> But that''s the problem, ''B'' is just an
"alphabet_partner" in this
> pairing table. There is nothing in the pairing table nor the 
> associations currently set that say b
> "B" is ALSO an "alphabet" which has an
"alphabet_partner" of ''A''.
> i.e. b.alpahbet_partner == []. it will NOT yield ''A''
Then make another association.  Remember, you can have multiple 
independent associations between the same pair of
tables.
> 
> I have tried this with my setup and thats the result i get but also 
> thinking about it makes me believe this can only be the end result.
With the setup you have, you''re right.  But adding one more set of 
associations should solve that.

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
Posted via http://www.ruby-forum.com/.

Hi again, thanks so much for your help i really appreciate it. I wanted 
to write another association like you said before but there is no 
Alphabet_Partner class to write it in. Its just a self reference to 
Alphabet table.

Alphabet_Partner is only defined in

class Alphabet < ActiveRecord::Base
  belongs_to :category
  has_many :pairings
  has_many :alphabet_partners,
       :through => :pairings

so after googling today im wondering how do i write the equivalent

of

classs Alphabet_Partner < AR:B

has_many :pairings
has_many :alphabets,:through => :pairings


The main reason why im trying to keep only one record in pairings is to 
make it easier to attach comments. Under my current setup If users 
comment on the pairing ab then id have to attach those comments to ba as 
well. The same goes for removing comments. I cant find any tutorials 
dealing with this. The only ones availalbe are for social networks and 
friendships. But no one ever comments on a friendship like "Oh they make 
such a lovely couple!" ;-) so im wondering how its done. And i do need 
the ability to do a.alphabet_partners and b.alphabet_partners.





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

Adam Akhtar wrote:
> Hi again, thanks so much for your help i really appreciate it. I wanted 
> to write another association like you said before but there is no 
> Alphabet_Partner class to write it in. Its just a self reference to 
> Alphabet table.
[...]

You don''t need an AlphabetPartner class.  You just need a second 
association between Alphabet and Pairing on the alphabet_partner_id 
column.  See the docs or the associations guide for how to do this.

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org
-- 
Posted via http://www.ruby-forum.com/.