validation_and_constraints.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

require 'active_record'
require 'rspec/autorun'
require 'pg'
require 'byebug'

# Extend for effect
class String
  def colorize(color_code)
    "\e[#{color_code};5m#{self}\e[0m"
  end

  def red = colorize(31)
end

# How to run this code:
# 1. chmod 755 postgres_errors.rb
# 2. ./postgres_errors.rb -fd

# The purpose of this example is to determine how ActiveRecord
# and ActiveModel report validation errors at the class (Model) level
# and how Postgres reports violations at the database level.
#
# Note: it's surely possible to extract all of this information from
# the documentation, but creating a demo or model might while be a lot
# faster than digging through multiple websites for the same information.

# Use postgres, because we want to investigate how constraint
# errors are raised from pg. Given running a local pg instance,
# `createdb scratch` possibly with an appropriate user name.
DB_SPEC = {
  adapter: 'postgresql',
  encoding: 'unicode',
  database: 'scratch',
  username: 'postgres'
}.freeze
ActiveRecord::Base.establish_connection(DB_SPEC)

FIRST_NAME_LENGTH = 16

# Example migration for custom validation.
class Orders < ActiveRecord::Migration[7.0]
  def change
    create_table :orders do |t|
      t.string :first_name # use Rails validations
      t.string :last_name, null: false, limit: FIRST_NAME_LENGTH, foo: :bar
    end
  end
end
Orders.migrate(:up) unless Orders.data_source_exists?(:orders)

# The usual base class for ActiveRecord models
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

# Descriptive comment for reek
class Order < ApplicationRecord
  # Validating in the Rails model may save a trip to the database, which
  # consumes a connection (probably) and induces network latency if
  # there is an error. On the other hand, if there are no errors,
  # validating in the Rails model creates overhead in the application.
  #
  # One valid question is: what's cheaper to scale, the application call
  # stack, or database connections?
  validates :first_name, presence: true, length: { maximum: FIRST_NAME_LENGTH }
end

RSpec.describe Order do
  let(:first_name) { 'foo' }
  let(:last_name) { 'bar' }
  let(:params) do
    {
      first_name:,
      last_name:
    }
  end

  subject(:order) { Order.new(params) }

  describe 'Rails validations' do
    context 'when valid' do
      example 'model passes with correct parameters' do
        expect(order.valid?).to be true
      end

      example "model is valid when 'last_name' is nil #{'(Danger!!!)'.red}" do
        params[:last_name] = nil
        expect(order.valid?).to be true
      end
    end

    context 'when invalid with Rails' do
      it 'raises an ActiveRecord error for long first_name' do
        params[:first_name] = first_name * 6

        expect do
          order.save!
        end.to raise_error(ActiveRecord::RecordInvalid)
           .with_message('Validation failed: First name is too long (maximum is 16 characters)')
      end

      it '".save!" raises an ActiveRecord error for long first_name' do
        params[:first_name] = first_name * 6

        expect do
          order.save!
        end.to raise_error(ActiveRecord::RecordInvalid)
           .with_message('Validation failed: First name is too long (maximum is 16 characters)')
      end

      it '".save" does not raise an ActiveRecord error for long first_name' do
        params[:first_name] = first_name * 6

        expect(order.save).to be false
      end
    end
  end

  describe 'PostgreSQL constraints' do
    context 'when violates postgres constraints' do
      it '".save!" raises a postgres violation on long first_name' do
        params[:last_name] = last_name * 6

        expect do
          order.save!
        end.to raise_error(ActiveRecord::ValueTooLong)
          .with_message(/PG::StringDataRightTruncation: ERROR:  value too long for type character varying\(16\)/)
      end

      it '".save" raises postgres violation when last_name is missing' do
        params[:last_name] = nil

        expect do
          order.save
        end.to raise_error(ActiveRecord::NotNullViolation)
          .with_message(/PG::NotNullViolation: ERROR:  null value in column "last_name" of relation "orders" violates not-null constraint/)
      end
    end
  end
end

# Fast and dirty way to reboot the table, causes an error when RSpec runs.
# Change the migration, uncomment the following line, run the script,
# comment out again, script will run with updated table.
# Orders.migrate(:down)
#
# Why do both?
# protects in different ways during data migration, during, backend updates,
# In a legacy code base, add for new column.
# For operator-driven updates