Comments on: Self-referential many-to-many relations in Ruby on Rails

By: Guillaume

Guillaume — Thu, 19 Jan 2012 02:27:08 +0000

Many many thanks!

By: christian

christian — Thu, 18 Feb 2010 09:57:31 +0000

still very usefull. having mine half way done, i’ve found yours and it helped a lot to get mine fully done. i’ve solved(hopefully?) the bidirectional thing by using not rails but ruby “magic”:

(roughly translated to your names in the user model)

def linked
self.linked_to | self.linked_from
end

By: guillaume

guillaume — Tue, 26 Aug 2008 21:01:34 +0000

Thanks a lot for your post, it helped me a lot !

By: guillaume

guillaume — Tue, 26 Aug 2008 20:56:42 +0000

Thanks a lot for your post

By: szeryf

szeryf — Fri, 23 Nov 2007 20:35:46 +0000

i didn’t use join, so I’m not sure which query you are referring to. your query returns the same as my query on view. nonetheless, I’m glad this helped you :)

By: provides 'brain'

provides 'brain' — Fri, 23 Nov 2007 17:26:49 +0000

great article, BTW, it really helped me a lot to understand rails relationships and self relationships and stuff. It worked great for a project in university.

thanks !

By: provides 'brain'

provides 'brain' — Fri, 23 Nov 2007 17:25:35 +0000

actually, it should be

select * from users where id in (select user_id from relationships where buddy_id = #{n} union select buddy_id from relationships where user_id = #{4});

By: provides 'brain'

provides 'brain' — Fri, 23 Nov 2007 15:55:05 +0000

Hey, you’r inner join doesn’t work for the ‘linked’ case, it just selects the ‘linked_from’.
art’s solution should work.

By: szeryf

szeryf — Sat, 27 Oct 2007 07:59:25 +0000

Yes, you can add additional fields to the relations table, but you cannot access them like this. The linked_to collection is a collection of User object and they don’t have feeling attribute. You have to use relation_to collection. Something like this:

@v.relation_to << @w, :feeling => ‘good’

By: Suryc

Suryc — Fri, 26 Oct 2007 15:41:11 +0000

Hey,
Is it possible :
– to store an other field ?

For example :
create_table :relationships, :id => false do |t|
t.column “user_id”, :integer, :null => false
t.column “buddy_id”, :integer, :null => false
t.column “feelling”, : string
end

– and to give the new paramter in :
@v.linked_to(feeling) << @w

Thx

By: “Self-referential many-to-many relations in Ruby on Rails” - Art at Code

Fri, 13 Jul 2007 12:34:35 +0000

[...] data makes me crazy I went on searching for any helpful information and found szeryf’s article. He uses some custom SQL to avoid [...]

By: szeryf

szeryf — Mon, 02 Jul 2007 18:04:31 +0000

art: yes and no :-) on the lower level we want to have directed relations in case we need to know who started the relation etc. but there are other situations when we want to treat the relationships as undirected (looking for a path between two users). that’s why I defined two versions of the relations.

as of your version of finding friends, I think it’s equivalent to my version. the difference is that you don’t use a view and instead do the union-trick inline in your query.

PS. thanks for first comment on my blog :)

By: art

art — Mon, 02 Jul 2007 16:28:06 +0000

I was just wondering…
aren’t the lists all the same?:
1. a list of users directly linked to (me → someone)
2. a list of users directly linked from (someone → me)
3. a list of users directly linked, no matter to or from (e.g. if A → B and B → C, then A and C should be on B’s linked list)

If this is so, you only need a find_friends method in your user model:
def self.find_friends(user_id)
find_by_sql(“SELECT users.*
FROM (
SELECT user_id, friend_id
FROM friendships
WHERE user_id = #{user_id}
UNION
SELECT friend_id, user_id
FROM friendships
WHERE friend_id = #{user_id}
) AS friends
INNER JOIN users
ON friends.friend_id = users.id
“)
end

please correct me if I’m wrong :)