main.rb

product_a = Product.fetch(:product_a).new(price: 0)
product_b = Product.fetch(:product_b).new(price: 0)

Product.fetch_class(:product_a).transaction do
  Product.fetch_class(:product_b).transaction do
    product_a.save!
    product_b.save!
  end
end

product.rb

class Product < ApplicationRecord
  self.abstract_class = true
  
  def self.register_class(key, klass)
    registry[key] = klass
  end
  
  def self.registry
    @registry ||= Hash::HashWithIndifferentAccess
  end
  
  def self.fetch_class(key)
    @registry.fetch(key)
  end
end

require_dependency 'product_a'
require_dependency 'product_b'

product_a.rb

class ProductA < Product
  register_class :product_a, self
  
  establish_connection :a
end

product_b.rb

class ProductB < Product
  register_class :product_b, self
  
  establish_connection :b
end

product_spec.rb

RSpec.describe Product do
  let(:product_a) { build :product_a }
  let(:product_b) { build :product_b }
  
  # write your test code by product_a, product_b
end

products.rb

FactoryBot.define do
  factory :product do
  end
  
  factory :product_a, class: 'ProductA' do
  end
  
  factory :product_a, class: 'ProductB' do
  end
end