arel – A relational algebra library – is the ruby library that that powers ActiveRecord. It provides a lower-level abstraction for working with SQL than ActiveRecord, and typically used when a particular query is not possible with ActiveRecord. Over the years, I’ve reached out for it on occasion. The definitive guide to Arel, the SQL manager for Ruby provides good information if you haven’t used it before.

I was recently working with a complicated query, and spent way more time because I assumed some things about it’s API that turned out to not be true.

Let’s first observe how ActiveRecord::Relations behave:

ar_relation = User.where.not(active_thru: nil)
ar_relation.to_sql
# =>  SELECT "users".* FROM "users" WHERE "users"."active_thru" IS NOT NULL

ar_relation.where(id: 5).to_sql
# => SELECT "users".* FROM "users" WHERE "users"."active_thru" IS NOT NULL AND "users"."id" = 5

ar_relation.to_sql
# SELECT "users".* FROM "users" WHERE "users"."active_thru" IS NOT NULL

Notice how calling a second where on a relation returns a new relations without modifying the original. This allows composing relations in Rails with great effect.

However, that is not true with Arel::SelectManager classes:

table = User.arel_table
arel_manager = table.where(table[:active_thru].not_eq(nil))
arel_manager.to_sql
# => SELECT FROM "users" WHERE "users"."active_thru" IS NOT NULL

arel_manager.where(table[:id].eq(5)).to_sql
# => SELECT FROM "users" WHERE "users"."active_thru" IS NOT NULL AND "users"."id" = 5

arel_manager.to_sql
# => SELECT FROM "users" WHERE "users"."active_thru" IS NOT NULL AND "users"."id" = 5

Notice how adding a new where clause modified the original object.

Caveat Emptor

The Statistics Handbook

I’ve been taking a statistics course in Coursera. The lectures and exercises are great, but I was really missing a text book that I can come back and reference. I was happy to find this gem: Free and available to download.

The definitive guide to Arel, the SQL manager for Ruby

Recently, I’ve been doing more complicated SQL queries in Rails, for which the ActiveRecord API is not enough. Enter Arel, a relational algebra library on which ActiveRecord is built, that allows more flexibility when using Rails. This is a great guide to using it. Arel is considered a private API in Rails. I’ve found it to be very stable, but be mindful when using it.

Introducing Tobox

This gem attempts to solve the write-to-multiple-databases problem when using background processing libraries in Ruby (e.g sidekiq). In effect, this is an event system, but without describing it heavily as such. The problem, stated more concisely is well described in Pattern: Transactional Outbox. The gem is new. I can’t comment on it’s maturity or stability. The author is right in pointing out that Rail’s ActiveJob DSL allows easy backgrounding of jobs, but ignores transactionality and dual-write problem that might exist. In fact, I was talking about this with some co-workers recently. One of the benefits of using GoodJob is that since the queue storage is in the same database, we can ignore this problem, as long as we are using a transaction.

A common pattern in Rails application to queue similar jobs for a collection objects. For example:

post.watchers.find_each do |user|
  NotifyOfChanges.perform_later(user, post)
end

The above will generate 1 INSERT SQL statement for each job queued. I recently noticed that GoodJob introduced a bulk enqueue feature. It allows using a single INSERT statement for all those jobs, similar to Rails’s #insert_all:

GoodJob::Bulk.enqueue do
  post.watchers.find_each do |user|
  NotifyOfChanges.perform_later(user, post)
end

Let’s see what the performance is locally:

class NoOpJob < ApplicationJob
  def perform
  end
end

require 'benchmark/ips'

Benchmark.ips do |x|
  x.config(:time => 10, :warmup => 5)


  x.report('Single Inserts') {
    ApplicationRecord.transaction do
      500.times { NoOpJob.perform_later }
    end
  }
  x.report('Bulk Inserts') {
    ApplicationRecord.transaction do
      GoodJob::Bulk.enqueue do
        500.times { NoOpJob.perform_later }
      end
    end
  }

  x.compare!
end

$ rails runner benchmark.rb
Running via Spring preloader in process 46655
Warming up --------------------------------------
      Single Inserts     1.000  i/100ms
        Bulk Inserts     1.000  i/100ms
Calculating -------------------------------------
      Single Inserts      0.833  (± 0.0%) i/s -      9.000  in  10.823196s
        Bulk Inserts      4.746  (± 0.0%) i/s -     48.000  in  10.155956s

Comparison:
        Bulk Inserts:        4.7 i/s
      Single Inserts:        0.8 i/s - 5.70x  slower

Locally, we can see a significant performance boost due to fewer round trips to the database. But using bulk enqueue can be even more impactful than that. Production systems typically see much more concurrent load that my local machine. When the queueing is wrapped in a transaction, it can be very disruptive. Long-running transactions can slow the whole system down. Bulk inserting records is a great way to keep transactions short, and the GoodJob feature provides an easy way to do that, while keeping the semantics of the code the same.

Making A Network Call: Mitigate The Risk

Nate Berkopec, well knows for his Ruby/Rails performance work, writes some good advice to mitigating the performance risk of making network calls: Make calls whenever possible in background jobs, set aggressive network timeouts, and use circuit breakers to fail fast when you detect a system is misbehaving.

I’m not saying this is easy, I’m saying it’s necessary.

Makefile Tutorial By Example

make is tried and true technology. I don’t write Makefiles often. When I do, having a mental model of how make treats dependencies helps make the whole enterprise more efficient and enjoyable. This guide has plenty of material to get you started.

Pure sh Bible

Very ingenious collection of recipes for sh, that avoid using new processes. Some of the syntax is clever, but terrifying to read. Case in point:

trim_string() {
    trim=${1#${1%%[![:space:]]*}}
    trim=${trim%${trim##*[![:space:]]}}

    printf '%s\n' "$trim"
}

What Is ChatGPT Doing … and Why Does It Work?

The Stephan Wolfram explains what ChaptGPT is doing. This article is very technical and on the long side. I found it quite enlightening to learn about what LLM are doing. A more accessible article article can be found, but I hate it’s name: “normie” is an objectionable term. It’s condecending, like you are not “in on something”.

I wish people would stop insisting that Git branches are nothing but refs

The article covers the friction of the mental model of using git vs the actual implementation. I’ve read a ton of complaints about git’s UX, the leaky abstraction, and all that. It’s true: Using git without knowing some of the plumbing is hard and doesn’t make much sense. In my experience, the more I’ve learned over the years from articles and especially from the Building Git Book, have made me much better at it, because I know understand the internals.

In any case, the author seems correct to me: Saying that branches are just refs, is not helpful. A branch is a moving ref, which implies a series of commits. That is how most people think and talk about branches.