snippet.rb

# Solved!

class Thing
  # these associations are fine
  belongs_to :user
  belongs_to :assigned_user, :class_name => "User", :child_key => [:assigned_user_id]
  
  # this is not working
  has n, :subscriptions
  has n, :subscribers, :through => :subscriptions, :class_name => 'User', :child_key => [:ticket_id], :remote_name => :user
end

class Subscription
  belongs_to :thing
  belongs_to :user
end

class User
  has n, :things
end

@thing = Thing.create

3.times do
  @thing.subscriptions.create :user => User.create
end

@thing.subscriptions.reload.length # => 3
@thing.subscribers.reload.length #=> 3