A two-part presentation given to the folks at EverTrue on the benefits of using ActiveRecord for database management in non-rails projects and a comparison of ActiveRecord and Arel and how Arel can be useful in building complex and interesting queries (inspired by a talk by Cameron Dutro)

1. ActiveRecord, Standalone
Migrations, and aRel
Alex Tironati

2. Overview
● Part 1 - ActiveRecord/Standalone Migrations
○ The Problem
○ Motivation
○ The Solution
○ Implementation
● Part 2 - Arel vs. AR
○ ActiveRecord Basics
○ ARel tables
○ Select and Join queries
○ Using a QueryBuilder
○ Bringing it home

3. Part 1
Standalone Database Migrations
in non-Rails Projects

4. The Problem

5. The Underlying Problem
● We have no process in place for managing
the database schemas for our Java projects
● This is the best
we could do
before:

6. Is that good enough?

7. Motivation
● We should be able to keep track of the schema
changes over time
● We should have a standard way to change the schema
across all databases
● We shouldn’t have to go through SQLPro to make those
changes
● We should have an easy way to recreate the current
schema from scratch
● We should be able to run those changes automatically,
after they are defined

8. What magical wonder-framework of Poseidon does all
this shit?
?

9. Oh that’s right

16. Implementation
● Add a .env file
● Make a project-level db folder
● Create a Gemfile and add:
○ gem 'active_record_migrations'
○ gem 'mysql2'
○ gem 'dotenv'
● Create a migration file for each table in your database:
○ rake db:new_migration name='create_note_tables'
○ Remember to include index definitions
○ Make sure all of the fields match your current schema exactly
○ run ‘rake db:migrate’ to test a lot locally
● When the migrations are written, go to new schema_migrations table

17. Implementation
● Create schema_migrations on staging and prod instances
● Duplicate entries (timestamps) from local schema_migrations table to
remote instances so that they match (the migrations are in order)
● run migrations on remote instance:
○ rake db:migrate RAILS_ENV=staging
● This will generate a file in db/schema.rb
○ Be sure to commit and push this to Github every time you run
migrations
● Update your Capistrano build to automatically run migrations for the
right environment on deploy

18. .env

19. Migration Example

20. Ugly way to make bigint primary key

21. Schema.rb

22. Capistrano task

23. Part 2
ActiveRecord vs. aRel

24. RailsConf 2014 - Advanced aRel: When
ActiveRecord Just Isn’t Enough
● Cameron Dutro
● Works on Twitter’s International
Engineering team
● https://www.youtube.com/watch?
v=ShPAxNcLm3o&index=14&list=PLE7tQUdR
KcyZ5jfnbS_osIoWzK_FrwKz5

25. Rails using SQL

26. Rails ¾ Improvements

27. Issues
● Just wrapping SQL in Rails methods
● You have to know underlying language
● No syntax checking
● Who likes matching question marks?
○ They’re confusing, man
● Not object-oriented

28. Arelized

29. Benefits
● Don’t have to know SQL
● Ruby syntax checking
● Chainable, composable
● No question marks
● Easier to read

30. ActiveRecord
● Database abstraction
● Persistence layer
○ Database rows as Ruby objects
● Domain Logic
○ Validations
● Associations

31. Arel
● “Relational Algebra” for Ruby
● Used to build queries,
generates ASTs
● Applies query optimizations
● Enables Chaining (subtrees)
● Not well documented
● Knows nothing about models or database, does not retrieve
data
○ That’s ActiveRecord’s job
○ Arel is all like “IDGAF” about yo models, db, or data

32. What’s an AST?

33. Arel Table

34. Arel-Helpers Gem
● gem install arel-helpers

35. Active Record Relations and .to_sql

36. Arelized

37. Select

38. More ActiveRecord vs. Arel
In this
case AR
is better/
cleaner

39. .not_eq(nil)

40. Reminder of the Data Model

41. Joins

42. Outer Joins Only Possible in Arel

43. Ugly Arel Version

44. Outer Joins Only Possible in Arel

45. Query Builder/Builder Pattern

46. Constructing a Query - Post Query Bldr

47. Constructing a Query cont.

48. Example in our own code!

49. Translated to SQL

50. The End