has_many :through

Self-referential has_many :through associations

create_table "nodes" do |t|
  t.column "name",      :string
  t.column "capacity",  :integer
end
create_table "edges" do |t|
  t.column "source_id", :integer, :null => false
  t.column "sink_id",   :integer, :null => false
  t.column "flow",      :integer
end

class Edge < ActiveRecord::Base
  belongs_to :source, :foreign_key => "source_id", :class_name => "Node"
  belongs_to :sink,   :foreign_key => "sink_id",   :class_name => "Node"
end
class Node < ActiveRecord::Base
  has_many :edges_as_source, :foreign_key => 'source_id', :class_name => 'Edge'
  has_many :edges_as_sink,   :foreign_key => 'sink_id',   :class_name => 'Edge'
  has_many :sources,  :through => :edges_as_sink
  has_many :sinks,    :through => :edges_as_source
end

22 comments — associations, rails

Comments

1. Jamie Quint – 2006-04-21 14:03:30

   This is very cool and just what I have been looking for. Thanks!

2. Andrew Viterbi – 2006-04-21 14:52:17

   Lets say each Node has_many :boxes. Is there a way to grab all the boxes that belong to a Node's sinks (or sources) in a single query?

   The model graph would look like this

   create_table "nodes" do |t|
   t.column "name", :string
   t.column "capacity", :integer
   end
   

   create_table "edges" do |t|
   t.column "source_id", :integer, :null => false
   t.column "sink_id", :integer, :null => false
   t.column "flow", :integer
   end
   

   create_table "boxes" do |t|
   t.column "node_id", :integer, :null => false
   t.column "label", :string
   end
   

   class Edge < ActiveRecord::Base
   belongsto :source, :foreignkey => "sourceid", :classname => "Node"
   belongsto :sink, :foreignkey => "sinkid", :classname => "Node"
   end
   

   class Node < ActiveRecord::Base
   hasmany :edgesassource, :foreignkey => 'sourceid', :classname => 'Edge'
   hasmany :edgesassink, :foreignkey => 'sinkid', :classname => 'Edge'
   hasmany :sources, :through => :edgesas_sink
   hasmany :sinks, :through => :edgesas_source
   has_many :boxes
   end

3. Justin – 2006-04-22 10:05:46

   Thank you for the many informative articles. I wish I had the foggiest notion what a digraph is, since then I would be able to actually understand how to create self-referential has_many :through associations. Without knowing how nodes, edges, sources, and sinks interrelate in the real world, I find that my feeble brain simply cannot grok what's going on here in the Rails world. :(

4. Peter Cooper – 2006-04-22 12:43:15

   I wonder if you could attach a polymorphic to this so you could also just get all edges associated, rather than just sources or sinks.. :)

5. Joel Hayhurst – 2006-04-23 11:19:05

   You can do bi-directional self-referencial associations with habtm as well, if that suits your needs.

   create_table 'parents_children', :id => false do |t|
       t.column 'parent_id', :integer, :null => false
       t.column 'child_id', :integer, :null => false
   end
   
   class Node < ActiveRecord::Base
       has_and_belongs_to_many :children,
           :class_name => 'Node',
           :join_table => 'parents_children',
           :foreign_key => 'parent_id',
           :association_foreign_key => 'child_id'
   
       has_and_belongs_to_many :parents,
           :class_name => 'Node',
           :join_table => 'parents_children',
           :foreign_key => 'child_id',
           :association_foreign_key => 'parent_id'
   end
   

6. Aslak Hellesoy – 2006-04-23 13:35:37

   I'm maintaining an act/plugin called acts_as_graph:

   http://dev.buildpatterns.com/trac/browser/rails/plugins/acts_as_graph

   It's based on habtm, but i'm considering basing it on hm :through.

   aslak

7. Henrik N – 2006-04-24 02:53:50

   Nice article, though I can't get this to work in my case. Extremely grateful for any help. I want to represent a not-necessarily-symmetrical friendship relation with attributes on the friendship relation. Code, followed by error message:

   create_table "friendships", :id => false, :force => true do |t|
     t.column "user_id", :integer, :null => false
     t.column "friend_id", :integer, :null => false
     t.column "created_at", :datetime
   end
   
   
   class Friendship < ActiveRecord::Base
   
     belongs_to :friendshipped,
       :foreign_key => "user_id",
       :class_name => "User"
     belongs_to :befriendshipped,
       :foreign_key => "friend_id",
       :class_name => "User"
   
   end
   
   
   class User < ActiveRecord::Base
   
     has_many :friendships,
       :foreign_key =>       'user_id',
       :class_name =>        'Friendship'
     has_many :befriendships,
       :foreign_key =>       'friend_id',
       :class_name =>        'Friendship'
   
     has_many :friends,
       :through =>         :friendships
     has_many :befrienders,
       :through =>         :befriendships
   
   end
   

   There are also fixtures that have populated the friendships table with some data. Error message:

   >> User.find(1).friends
   ActiveRecord::HasManyThroughSourceAssociationMacroError: ActiveRecord::HasManyTh
   roughSourceAssociationMacroError
           from c:/Program/Ruby/lib/ruby/gems/1.8/gems/activerecord-1.14.2/lib/acti
   ve_record/reflection.rb:181:in `check_validity!'
           from c:/Program/Ruby/lib/ruby/gems/1.8/gems/activerecord-1.14.2/lib/acti
   ve_record/associations/has_many_through_association.rb:6:in `initialize'
           from c:/Program/Ruby/lib/ruby/gems/1.8/gems/activerecord-1.14.2/lib/acti
   ve_record/associations.rb:876:in `friends'
           from (irb):82
   >> "friendship".pluralize
   => "friendships"
   

8. Henrik N – 2006-04-24 03:13:52

   I should add that user.friendships, user.befriendships, friendship.friendshipped and friendship.befriendshipped work fine - but user.friends and user.befrienders errors.

   After restarting the server and the console entirely, I get ActiveRecord::HasManyThroughSourceAssociationNotFoundError: ActiveRecord::HasManyThroughSourceAssociationNotFoundError instead. Still an error, though.

9. Henrik N – 2006-04-24 03:23:51

   Sorry for the repeated comments. This should be the last one.

   Solved it. Hope this helps someone else. Apparently you should do this:

   has_many :friends,
       :through =>     :friendships,
       :source =>  :befriendshipped
   
   has_many :befrienders,
       :through =>     :befriendships,
       :source =>  :friendshipped
   

10. Bill – 2006-04-26 02:40:41

    Thanks! I was trying to work out how to do exactly this and I think you may have just saved me a lot of time.

11. befuddled – 2006-05-15 17:12:31

    So I started trying to figure out how my models would interact, and I ended up with something that looked a little like an icosahedron, with all of the edges being two-way habtm relationships. I just assumed I was doomed.

    I don't yet understand what you wrote above, but if I get the gist of it, it looks like what I need. Maybe.

    Are :source, :sink, :edgesas_source, and :edgesas_sink the join models?

    How is eager loading affected, when there are multiple ':through's going on?

    I.e.

    A has_many :B, :through E (and possibly vice versa)

    B has_many :C, :through F (and possibly vice versa)

    C has_many :D, :through G (and possibly vice versa)

    Aren't I still hosed if I want to look at A.B.C.D? Or C.B.A? Or does your trick above make it work?

12. Josh Susser – 2006-05-15 17:31:14

    It sounds like you don't need a self-referential scheme, just a lot of associations. Unless you need to have an A refer to an A (by whatever association name), then you don't need this trick.

    Eager loading works with :through associations, and it shouldn't matter that you have multiple associations in your model. I haven't used eager loading through multiple steps of :through associations though, so I can't say with confidence that works.

    I'm scared of your icosahedron!

13. Dan Kirkwood – 2006-07-05 10:38:51

    Josh, many thanks for the excellent article. I'd struggled for a couple of days to get this working -- works like a charm now, even thru STI! I missed the source->sink switch on first read: hasmany :sources, :through => :edgesas_sink hasmany :sinks, :through => :edgesas_source

    Just wanted to highlight that part in case it causes someone else trouble...

14. Skiz – 2006-07-16 11:22:20

    Henrik N, Thank you for posting your information and resolution. This is exactly what I was looking for and was having the same issue.

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16. tobywan – 2006-09-25 17:58:57

    I thank you for this Great explanation of how to set up models self-referentially, and the db, BUT. . .can you give examples of how you access the various objects. . .once this is all set up? that's where i am now stuck. . .

17. tobywan – 2006-09-25 17:59:01

    I thank you for this Great explanation of how to set up models self-referentially, and the db, BUT. . .can you give examples of how you access the various objects. . .once this is all set up? that's where i am now stuck. . .

18. greatcpy – 2006-11-15 04:54:55

    this is simply a shortcut not a direct soln check the pdfs has_many :friends, :through => :friendships, :source => :befriendshipped

    has_many :befrienders, :through => :befriendships, :source => :friendshipped

19. Steve Ehrenberg – 2006-11-24 22:05:08

    I've been racking my braing trying to get a self-referential friends list working. Here's what I have.

     create_table :friendships, :id => false do |t|
       t.column :user_id,    :integer, :null => false
       t.column :friend_id,  :integer, :null => false
       t.column :status,     :integer, :null => false, :default => 0
       t.column :created_at, :datetime
     end
    
     class Friendship < ActiveRecord::Base
       belongs_to :user, 
           :class_name => 'User', 
           :foreign_key => 'user_id'
       belongs_to :friend, 
           :class_name => 'User',
           :foreign_key => 'friend_id'
     end
    
     class User < ActiveRecord::Base
       has_many :friendships
       has_many :friends, :through => :friendships
     end
    
     class FriendsController < ApplicationController
       def add
         user = User.find(params[:user_id])
         friend = User.find(params[:id])
         user.friends << friend
         friend.friends << user
       end
     end
    
     http://localhost:3000/users/1/friends/2;add
    
     ActiveRecord::StatementInvalid (Mysql::Error: Column 'friend_id' cannot be null: INSERT INTO friendships ('status', 'user_id', 'created_at', 'friend_id') VALUES(0, 2, '2006-11-24 22:11:58', NULL)):
    

    Notice that 'userid' is trying to be set as 2 (which is what the friendid should be) which leads me to believe that I suck at has_many :through ... x=\

    I've tried pretty much copying and pasting some of the examples here and I get pretty much the same result. I've tried to move the foreignkey's around to different places. I've tried (in my eyes) just about everything. What else could be wrong? I know it has been some time since this original post and now, so things might have changed. Anyone know something I don't? I'm very new to hasmany :through but I just can't figure this out for the life of me. Any help would be GREATLY appreciated.

20. Steve Ehrenberg – 2006-11-24 23:00:44

    Just one more added piece of information for my above problem. I created a row in the friendships database to link 2 users, and it seems that FINDING the users friends (User.find(1).friends.find(2)) works fine. It's just creating and deleting that isn't working. I know that I could just manually create these things, but I'd really like to get it to work properly so I don't have worry about anything breaking. Thanks again.

21. Frank Quednau – 2007-01-03 09:08:22

    Big, big thanks to this lovely article from my part. This was cracking my brain (I am quite new to RoR) and to find such concentrated knowledge here is wonderful (although it took me a while to find it with Google...). However, the presented solution did not save me the relationship. I suppose, because the relationship is itself a model that may have to be saved explicitly...? Either way, Joel Hayhurst's solution worked like a charm in my case (because, although my relationship can be clearly labelled, it has no additional attributes so far), so many thanks to Joel as well. Take care...

22. Maledictus – 2007-01-23 13:11:12

    Has anyone a link or an idea how to apply this for a not directed graph? (Self-referential, many-to-many, and with attributes for the edges). Many thanks!

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