Reading from a Temp Table that is Dropped on Commit

temp_table_read_examples.rb

# frozen_string_literal: true

# Temp Table Read Example
#
# Demonstrating how data can be read from a temp table
# that is nomrally dropped when a transaction is committed,
# without altering the transaction.
#
# cp <this file> <rails project root>
# cd <rails project w/ db>
# bundle exec rails runner temp_table_read_examples.rb
#

module Helpers
  def create_temp_table
    ActiveRecord::Base.connection.execute <<~SQL.squish
      create temp table my_temp (
        name varchar,
        number bigint
      )
      on commit drop
    SQL
  end

  def insert_into_temp_table
    ActiveRecord::Base.connection.execute "insert into my_temp values ('Bruce Springsteen', 1)"
  end

  def read_from_temp_table
    ActiveRecord::Base.connection.execute "select * from my_temp"
  end

  def temp_table_exists?
    ActiveRecord::Base.connection.execute "select * from my_temp"
    true
  rescue ActiveRecord::StatementInvalid
    false
  end

  def create_and_insert
    create_temp_table
    insert_into_temp_table
  end

  def create_and_insert_in_transaction
    ActiveRecord::Base.transaction do
      create_and_insert
    end
  end
end

module Examples
  include Helpers

  def read_attempt_after_transaction
    create_and_insert_in_transaction
    read_from_temp_table ## -- should fail.
  end

  def read_with_outer_transaction
    result = nil
    ActiveRecord::Base.transaction do
      create_and_insert_in_transaction
      result = read_from_temp_table ## -- should work.
    end
    result
  end

  def read_with_outer_transaction_and_rollback
    result = nil
    ActiveRecord::Base.transaction do
      create_and_insert_in_transaction
      result = read_from_temp_table ## -- should work.
      raise ActiveRecord::Rollback
    end
    result
  end
end

class TempTableShowcase
  include Examples

  def run
    puts "EXAMPLE 1: Reading from temp table after transaction should fail."
    begin
      read_attempt_after_transaction
    rescue ActiveRecord::StatementInvalid => e
      puts "-- Assertion 1 Success: received ActiveRecord::StatementInvalid as expected"
      puts "-- because table was dropped when transaction was committed."
      puts "-- #{e.message}"
    end

    puts "\nEXAMPLE 2: Reading from temp table within outer transaction should succeed."
    result = read_with_outer_transaction
    puts "-- Assertion 1 Success: received #{result.to_a.inspect} as expected" if result
    if temp_table_exists?
      puts "-- Assertion 2 Failed: temp table was NOT dropped when outer transaction was committed."
    else
      puts "-- Assertion 2 Success: temp table was dropped as expected."
    end

    puts "\nEXAMPLE 3: Reading from temp table within outer transaction that is rolled back should succeed."
    result = read_with_outer_transaction_and_rollback
    puts "-- Assertion 1 Success: received #{result.to_a.inspect} as expected" if result
    if temp_table_exists?
      puts "-- Assertion 2 Failed: temp table was NOT dropped when outer transaction was rolled back."
    else
      puts "-- Assertion 2 Success: temp table was dropped as expected."
    end
  end
end

TempTableShowcase.new.run