Q:

Creating a simple many-to-many self-join association in Rails

I'm trying to create a simple many-to-many association for my Artist model. I've tried following the example here but for some reason its just not translating for my particular project. I basically just want users to belong to groups and reflect their memberships in my show view.
<!-- Shows the list of groups an artist belongs to -->

<% if @artist.groups.any? %>
  <%= @artist.groups.each do |group| %>
    <%= group.name %>
  <% end %>
<% end %>

<!-- Shows the members of the groups -->

<% if @artist.members.any? %>
    <p>
      <b>Members:</b>
      <% @artist.members.each do |member| %>
        <%= member.name %>
      <% end %>
    </p>
<% end %>

I currently have my User model setup with a has_one association and works great. Now I just need to make it into a many_to many association to be able to call @artist.groups. vs. @artist.group. Would a a join table now be necessary to get this to work?
class Artist < ActiveRecord::Base
  attr_accessor :group_id      

  has_many :members, class_name: 'Artist', foreign_key: 'group_id'
  belongs_to :group, class_name: 'Artist', foreign_key: 'group_id'
end

A:

You definitely need a join table for a many-to-many association.
Try something like this (this is from memory so may need some adjusting):
class Membership
  # attributes "artist_id" and "group_id"
  belongs_to :member, class_name: "Artist"
  belongs_to :group, class_name: "Artist"
end

class Artist
  # no foreign keys required
  has_many :group_memberships, class_name: "Membership", foreign_key: :group_id
  has_many :members, through: :group_memberships

  has_many :memberships, foreign_key: :member_id
  has_many :groups, through: :memberships
end

Then your view can stay the same as it is in the question.
You probably also want to add a unique index to your memberships table:
add_index :memberships, :member_id
add_index :memberships, :group_id
add_index :memberships, [:member_id, :group_id], unique: true

... with the appropriate validates_uniqueness_of validation in your Artist model.

