How to Conditionally Rollback a Transaction Using the PG Ruby Gem - ruby

Question

I am currently taking a databases course and one of the lab questions has me stumped as to how to implement the above and indeed, if it was even possible. I have tried searching the docs but the transaction method defined is quite vague.
This is the first time I have tried to do any database manipulation WITHOUT the comfort blanket of Rails and so I am a little lost. I have managed to create a connection to my postgresql database and can execute statements, the final thing that I need to do is rollback a transaction based on a simple condition.
Allow me to show you the code:
require 'pg'
#conn = PG::Connection.open(:dbname => 'db_15_11_labs')
#conn.prepare('insert', 'INSERT INTO house (housenumber, street) VALUES ($1, $2) returning id')
#words = ["Foo", "Bar", "Bash", "Bang"]
def populate
100.times.each_with_index do |i|
# cycle through the #words array for street names, use i as house number
ins = #conn.exec_prepared('insert', [i, "#{#words1[i % 4]} street"])
end
end
Basically, the condition is that if the returned id (ins[0]['id']) is even, rollback the transaction. I figure that if ins[0]['id'] % 2 == 0 I need to throw some sort of exception but how do I do that and more importantly, how do I encode that information into the syntax given in the docs?
Thank you for your time.
EDIT 1
I have now managed to get the syntax that will allow for the conditions to be placed within the defined transaction as follows:
#conn.transaction do |conn|
ins = #conn.exec_prepared('insert', [i, "#{#words1[i % 100]} street"])
end
and so this question really becomes more "how do I throw and catch an exception when ins[0]['id'] % 2 == 0
I have tried following a couple of simple tutorials on raising exceptions but doing the following:
throw :id_exception if (#ins[0]['id'].to_i % 2) == 0
catch :id_exception do
puts "caught :id_exception #{}"
end
inside the transaction results in an 'throw': uncaught throw :id_exception (ArgumentError)

Answer 1

Transactions rollback automatically if an exception is raised inside the block. So you just need to create a little Exception class for yourself, raise one inside the block, and then arrange to rescue it so your program doesn't error-exit due to the raised exception.
Try something like this:
class MyLittleIdError < StandardError
end
begin
#conn.transaction do |conn|
ins = conn.exec_prepared('insert', [i, "#{#words1[i % 100]} street"])
raise MyLittleIdError if (ins[0]['id'].to_i % 2) == 0
end
rescue MyLittleIdError
puts "aha! we have rolled back."
end