Many-to-many associations in a single model (rails)

Question

Lets say I have the following table of Users

id, type
1, "A"
2, "B"
3, "B"
4, "A"
5, "B"
6, "A"

There are two user types. "A" users, and "B" users. "A"s can be connected to many "B"s, and "B"s can be connected to many "A"s. Let's say we have the following 'connections' table

id, A_id, B_id

1, 1, 2
2, 4, 2
3, 4, 3
4, 6, 5
5, 1, 5
6, 4, 5

Which would represent the following graph:

connections graph

I could have an "A"s table that stores the foreign_key indexes of the users with the "A" type (i.e. 'these users are type "A"), and similarly for "B", in which case I could just define a simple has_many association through the connections table from "A" to "B" and vice versa.

I want to be able to type something like a.b to produce all the "B" that the 'a' is connected to, as well as b.a to produce all the "A" that the 'b' is connected to.

My question is: can this many-to-many association between "A" and "B" be defined using a single User model instead? Can a self-join be used?

Thank you for your time

Does this [Railscast](http://railscasts.com/episodes/163-self-referential-association) go in the direction you're looking for? If not, what could serve as a real-world example for your model object and the relationships between them? — Thomas Klemm, May 22 '13 at 23:35
possible duplicate of [How do rails association methods work?](http://stackoverflow.com/questions/1529606/how-do-rails-association-methods-work) — user2205763, Aug 10 '14 at 06:45

score 0 · Accepted Answer · answered May 23 '13 at 00:49

Looks like a pretty standard example of has_many :through and STI.

Given:

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :type

      t.timestamps
    end
  end
end

class CreateConnections < ActiveRecord::Migration
  def change
    create_table :connections do |t|
      t.integer :a_user_id
      t.integer :b_user_id

      t.timestamps
    end
  end
end

class User < ActiveRecord::Base
end

class AUser < User
  has_many :connections
  has_many :b_users, :through => :connections
end

class BUser < User
  has_many :connections
  has_many :a_users, :through => :connections
end

class Connection < ActiveRecord::Base
  belongs_to :a_user
  belongs_to :b_user
end

When (rails console output snipped for brevity):

>> a_user = AUser.create!
>> b_user = BUser.create!
>> connection = a_user.connections.build
>> connection.b_user = b_user
>> connection.save!

Then:

>> a_user.b_users
=> ActiveRecord::Associations::CollectionProxy of BUsers

If your AUser and BUser objects will have different sets of attributes then you should use Polymorphic Associations instead, which requires creating a couple more tables but really doesn't complicate things much.

Many-to-many associations in a single model (rails)

1 Answers1