# Representing relationships between model objects **URL:** \ **Category:** Rails\ **Created:** [February 26, 2006, 7:21pm UTC](https://www.ruby-forum.com/t/representing-relationships-between-model-objects/73869 "2006-02-26T19:21:53Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![SBSBSB](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ecccb3/32.png) [@SBSBSB](https://www.ruby-forum.com/u/SBSBSB)\ **Post date:** [February 26, 2006, 7:21pm UTC](https://www.ruby-forum.com/t/representing-relationships-between-model-objects/73869/1 "2006-02-26T19:21:53Z")
I’m a little stumped on how to approach the subject of representing relationships between a single model object. Basically, I want a User to be able to mark others as Friends, Contacts, etc. The caveat is that User A can mark User B as a Friend but that doesn’t mean User A is marked as a friend by User B. It can either be a one-way relationship or two-way relationship. Right now I’m confused as to whether there is a “proper” way to do this. I’m looking at Single Table Inheritance, HABTM, and now the new Polymorphic Associations in Edge Rails. The main thing that’s confusing me is that when marking someone as Friend, you will have to reference the “user\_id” of both User A and User B with a foreign key, but I don’t know how to sort that out. Sorry if this question is vague but my understanding of accomplishing this in rails is even vaguer. However, most of the examples I see involve defining relationships between two different models such as Products, LineItems, and Orders or something like that. Sam ---
**Author:** ![SBSBSB](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ecccb3/32.png) [@SBSBSB](https://www.ruby-forum.com/u/SBSBSB)\ **Post date:** [February 27, 2006, 2:54am UTC](https://www.ruby-forum.com/t/representing-relationships-between-model-objects/73869/2 "2006-02-27T02:54:11Z")
Hi ! 2006/2/26, SB [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Basically, I want a User to be able to mark others as Friends, > Contacts, etc. The caveat is that User A can mark User B as a Friend > but that doesn’t mean User A is marked as a friend by User B. It can > either be a one-way relationship or two-way relationship. I would go with a HABTM relationship, but with a full model in the middle. class User \< ActiveRecord::Base has\_many :relationships def make\_friend\_of!(name) target = User.find\_by\_name(name) raise “No person by that name” unless target ``` self.relationships.create(:target => target) ``` end end class Relationship belongs\_to :target end user = User.new(:name =\> ‘Francois’) user.make\_friend\_of!(‘John’) That’s the way I would implement it. Of course, I don’t know all your requirements, so I can’t really say it is the best way. Hope that helps ! ---
**Author:** ![SBSBSB](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ecccb3/32.png) [@SBSBSB](https://www.ruby-forum.com/u/SBSBSB)\ **Post date:** [February 27, 2006, 3:15am UTC](https://www.ruby-forum.com/t/representing-relationships-between-model-objects/73869/3 "2006-02-27T03:15:27Z")
That’s similar to what I was thinking but then I don’t understand how you can map clarify that one User is the owner of the relationship and the other User is classified under the relationship because linking them by foreign\_key, they would both be “user\_id”. After sleeping on it, I’m thinking of going with two Single Table Inheritances. One is Friends and Another is Groups. Friends will have various “types” and each User can aggregate their Friends in Groups. It would function as a “join table” but allow more features. That way I can model one way relationships. Such as: John =\> Mary’s Friend Mary =\> John’s Contact Polymorphic Associations look promising but I’ll need to see more documentation before I can even explore it as an option. Thanks for the response! Sam ---
**Author:** ![SBSBSB](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ecccb3/32.png) [@SBSBSB](https://www.ruby-forum.com/u/SBSBSB)\ **Post date:** [February 27, 2006, 3:38am UTC](https://www.ruby-forum.com/t/representing-relationships-between-model-objects/73869/4 "2006-02-27T03:38:03Z")
You can do this in several ways. The trick is that you can tell Rails to use a different name for the field instead of the default one based on the model class. That will support one-way relationships, or two-way if you make links in both directions There is a good description of how to do that at the bottom of this wiki page: [http://wiki.rubyonrails.org/rails/pages/HowToCreateASelfReferentialManyToManyRelationship](http://wiki.rubyonrails.org/rails/pages/HowToCreateASelfReferentialManyToManyRelationship) If you are brave enough to try Edge Rails, you can do something similar with the has\_many :through association, otherwise known as join models. This is probably the way to go if you want to pursue your open-ended set of relationship types. The schema-safe way to do that is to set up a 3-way join between user1, user2, and call it a role. The role table would have a name and maybe other fields that describe things about the relationship. That way you can add new roles to the app without having to create a new model class or modify the schema. Hmm, I should probably write something up about how to do this with some code examples. –josh SB wrote: > That’s similar to what I was thinking but then I don’t understand how > you can map clarify that one User is the owner of the relationship and > the other User is classified under the relationship because linking > them by foreign\_key, they would both be “user\_id”. > > After sleeping on it, I’m thinking of going with two Single Table > Inheritances. One is Friends and Another is Groups. Friends will > have various “types” and each User can aggregate their Friends in > Groups. It would function as a “join table” but allow more features. > That way I can model one way relationships. > > Such as: > John =\> Mary’s Friend > Mary =\> John’s Contact > > Polymorphic Associations look promising but I’ll need to see more > documentation before I can even explore it as an option. > > Thanks for the response! > Sam