polymorp.rb

# Suppose the AccountHolder parent class is used
# to simply have an association with an account so
# both corporations and people can have accounts.

class Account < ActiveRecord::Base
  belongs_to :account_holder
end

class AccountHolder < ActiveRecord::Base
  has_one :account
end

class Corporation < AccountHolder
end

class Person < AccountHolder
end

# Just use a polymorphic association instead

class Account < ActiveRecord::Base
  belongs_to :account_holdable, polymorphic: true
end

class Corporation < AccountHolder
  has_one :account, as: :account_holdable
end

class Person < AccountHolder
  has_one :account, as: :account_holdable
end

# As such, accounts would need the polymorphic columns
# `account_holdable_type` and `account_holdable_id` which
# would describe the class as well as the primary key, respectively.

class CreateAccounts < ActiveRecord::Migration[5.0]
  def change
    create_table :accounts do |t|

      # ... various fields
      
      t.integer :account_holdable_id
      t.string  :account_holdable_type
      
      t.timestamps
    end
  end
end