Single Table Inheritence rails model that can behave as different types

Question

I'm having trouble with a single table inheritence (STI) data model where a model can be one of the types OR both. I can add a third type, which would be something like TradingCompanyClearingMember but then I need to duplicate code from both models into the 3rd. Where this becomes especially difficult is defining polymorphic relationships. Is there an easier / better way / alternatives to consider to model this behavior?

class Company < ActiveRecord::Base
  validates :type, inclusion: %w(TradingCompany ClearingMember TradingCompanyAndClearingMember)
end

class TradingCompany < Company
  has_many :traders, as: :tradeable
  belongs_to :clearing_member
end

class ClearingMember < Company
  has_many :trading_companies
  has_many :contacts
end

class TradingCompanyAndClearingMember < Company
  # copied clearing member relationships
  has_many :trading_companies
  has_many :contacts
  # copied trading company relationships
  has_many :traders, as: :tradeable
  belongs_to :clearing_member
end

class Trader
  belongs_to :tradeable, polymorphic: true
end

Answer 1

The closest thing ruby has to multiple inheritance is mixing in modules. You might be able to put the TradingCompany code into a TradingCompanyModule which you import into both trading company type classes. But I think you're probably better served making Company have one TradingCompany and one ClearingMember instead of using inheritance.