has_many_through_across_databases.rb

class ActiveRecord::Base
  def self.table_name_prefix
    "appname."
  end
end

class TransactionalBase < ActiveRecord::Base
  self.abstract_class = true
  establish_connection "#{RAILS_ENV}-transactional"
  
  def self.table_name_prefix
    "appname_transactional."
  end
end

# Now you can do JOINs across two databases on the same host
# provided your permissions are set up appropriately

class Hospital < AcitveRecord::Base
  has_many :hospital_doctors
  has_many :doctors, :through => :hospital_doctors
end

class HospitalDoctor < ActiveRecord::Base
  belongs_to :hospital
  belongs_to :doctor
end

class Doctor < TransactionalBase
  has_many :hospital_doctors
  has_many :hospitals, :through => :hospital_doctors
end


Example:
> Hospital.first.doctors 
Will use the following SQL:
SELECT `appname_transactional`.`doctors`.* FROM `appname_transactional`.`doctors` INNER JOIN `appname`.`hospital_doctors` ON `appname_transactional`.`doctors`.id = `appname`.`hospital_doctors`.doctor_id WHERE ((`appname`.`hospital_doctors`.hospital_id = 1))