class Buyer < ActiveRecord::Base
  has_many :leads
  has_many :zips, :through => :leads

class Zip < ActiveRecord::Base
  has_many :leads
  has_one :buyer, :through => :leads 

class Lead < ActiveRecord::Base
  belongs_to :zip
  belongs_to :buyer


vs.

class Buyer < ActiveRecord::Base
  has_many :zips
  has_many :leads, :through => :zips

class Zip < ActiveRecord::Base
  has_many :leads
  belongs_to :buyers 

class Lead < ActiveRecord::Base
  belongs_to :zip