If a Rails server is running in single threading mode, you can begin a transaction by a request / action, then end a transaction with another request / action. All requests between these two requests will be wrapped in one transaction. How can this be useful? Sometimes.

begin_and_end_transactions_by_requests.rb

module Concerns
  module Transactions
    extend ActiveSupport::Concern

    def transaction_begin
      ActiveRecord::Base.connection.begin_db_transaction
      ActiveRecord::Base.connection.increment_open_transactions
      render nothing: true
    end

    def transaction_rollback
      ActiveRecord::Base.connection.decrement_open_transactions
      ActiveRecord::Base.connection.rollback_db_transaction
      render nothing: true
    end

  end
end