Multiple foreign keys to the same type of table

rubyonrails-talk
1

I'm not sure how to have two foreign keys of the same type.

e.g.

class CreateFriends < ActiveRecord::Migration   def self.up     create_table :friends do |t|     t.column :user_id, :integer     t.column :user_id, :integer     t.column :ready, :boolean     end   end

  def self.down     drop_table :friends   end end

Now I can't have the same name twice, but if i change one of them to user2_id won't this break the naming convention.

Any ideas?

The api docs for ActiveRecord::Associations::ClassMethods describe how you specify associations between your models, including overriding the default foreign key and class name when you don't follow rails' naming conventions.

Is this a bad idea anyway?

There are lots of sound reasons for referring to the same table multiple times from a single row.

It _could_ also mean that your FK is in the wrong table, if you're just trying to model a one-to-many relationship.

Isak

2

Just guessing from your model names but it appears that you are building a "reflexive many-to-many relationship." Reflexive meaning that you are creating a join table between two rows of a single table:

User <------->> Friend <<-------> User

An excerpt from "Rails Recipies" shows this type of Self Referential Many-to-Many Releationship:

class AddPeopleAndTheirFriendsRelationship < ActiveRecord::Migration   def self.up     create_table :people do |t|       t.column "name", :string     end     create_table :friends_people, :id => false do |t|       t.column "person_id", :integer       t.column "friend_id", :integer     end   end

  def self.down     drop_table :people     drop_table :friends_people   end end

and the Person model looks like:

class Person < ActiveRecord::Base   has_and_belongs_to_many :friends,     :class_name => "Person",     :join_table => "friends_people",     :association_foreign_key => "friend_id",     :foreign_key => "person_id" end

3

By the way, I would likely model this with the join table as a separate resource using has_many_through. I would call the joining resource Friend. This way you can store information about the association in the Friend model. Such as in your case with :ready => boolean. Or things like friendship_established_on and other such details.