gistfile1.rb

# This works, right now:

class EmailCaptureCampaign < ActiveRecord::Base
  has_many :campaign_merchants, :as => :campaignable, :dependent => :destroy
  has_many :merchants, :through => :campaign_merchants
end

class Merchant < ActiveRecord::Base
  has_many :campaign_merchants, :dependent => :destroy
  has_many :campaigns, :through => :campaign_merchants, :source => :campaignable, :source_type => 'EmailCaptureCampaign'
end

class CampaignMerchant < ActiveRecord::Base
  belongs_to :campaignable, :polymorphic => true
  belongs_to :merchant
end

# I want to add a new kind of campaign:

class UserAcquisitionCampaign < ActiveRecord::Base
  has_many :campaign_merchants, :as => :campaignable, :dependent => :destroy
  has_many :merchants, :through => :campaign_merchants
end

# but the :source_type => 'EmailCaptureCampaign' in Merchant above disallows this

I want to have merchant.campaigns return a collection of all campaigns, which will be of type UserAcquisitionCampaign or EmailCaptureCampaign.