Tropical on Rails 26' | Modeling Class Hierarchies in Active Record

Single-table inheritance and purely abstract classes work… until they don’t.

So where do you turn when things start to get messy?

In this talk, we break down single-table inheritance, purely abstract classes, and delegated types, discussing when each makes sense, where they fall short, and how to choose the right tool for the job.

Transcript

1. Modeling Class Hierarchies in Active Record Nicolas Erlichman CTO at GoGrow
2. Tropical on Rails Why I really like modeling & polymorphism
3. Tropical on Rails Why I really like modeling & polymorphism It can get messy
4. Tropical on Rails Why I really like modeling & polymorphism It can get messy Active Record offered only two ways to do inheritance
5. Tropical on Rails Why I really like modeling & polymorphism It can get messy Active Record offered only two ways to do inheritance Delegated Types is still not widely known/used
6. Tropical on Rails Survey
7. Tropical on Rails Survey
8. Tropical on Rails Survey
9. Tropical on Rails Survey Not for me
10. Tropical on Rails Survey Results
11. Tropical on Rails Survey Results
12. Tropical on Rails Survey Results
13. Tropical on Rails Survey Results
14. It is a pattern in Rails that allows multiple models to be stored in a single database table. Ruby on Rails Guides SINGLETABLE INHERITANCE
15. Tropical on Rails Single-Table Inheritance Car make model body_style Truck make model bed_type
16. Tropical on Rails Single-Table Inheritance Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type
17. Tropical on Rails Single-Table Inheritance Vehicle make, null: false model, null: false type, null: ? body_style bed_type Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type Car Truck
18. Tropical on Rails Benefits Simple to setup
19. Tropical on Rails Benefits Simple to setup Unified queries
20. Tropical on Rails Benefits Simple to setup Unified queries Refactoring doesn't require additional migrations
21. Tropical on Rails Downsides Table bloat
22. Tropical on Rails Downsides Table bloat Awkward constraints
23. Tropical on Rails Downsides Table bloat Awkward constraints Hard to maintain
24. Tropical on Rails Job Posts Example
25. Class-table inheritance, where the superclass doesn't persist any attributes and all concrete subclasses must persist all the shared attributes themselves in their own tables. Rails Source Code PURELY ABSTRACT CLASSES
26. Tropical on Rails Example Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type
27. Tropical on Rails Example with Purely Abstract Classes Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type make, null: false model – null: false body_style, null: false Car Truck make – null: false model – null: false bed_type, null: false Vehicle
28. Tropical on Rails Benefits Clean schemas (no table bloat)
29. Tropical on Rails Benefits Clean schemas (no table bloat) Independent evolution
30. Tropical on Rails Benefits Clean schemas (no table bloat) Independent evolution Clean Ruby modeling
31. Tropical on Rails Downsides Harder cross-class queries
32. Tropical on Rails Downsides Harder cross-class queries Duplication & coordinated migrations
33. Tropical on Rails Downsides Harder cross-class queries Duplication & coordinated migrations Can’t belong to abstract class
35. Tropical on Rails Application Record Example
36. Tropical on Rails Gitlab’s User Mentions Example
37. Solves the STI problem of table bloat. This approach allows us to store shared attributes in a superclass table and have separate tables for subclass-specific attributes. Rails Source Code DELEGATED TYPES RAILS 6.1
38. Tropical on Rails Delegated Types Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type
39. Tropical on Rails Delegated Types body_style – null: false Car bed_type – null: false Truck Vehicle make – null: false model – null: false … Vehicle make, null: false model, null: false body_style, null: false Car bed_type, null: false Truck Car make model body_style Truck make model bed_type
40. Tropical on Rails Benefits Unified queries
41. Tropical on Rails Benefits Unified queries Clean schemas (no table bloat)
42. Tropical on Rails Benefits Unified queries Clean schemas (no table bloat) Scales naturally as new types appear
43. Delegated Types
44. Tropical on Rails Polymorphic Association
45. Tropical on Rails
46. Tropical on Rails
47. Tropical on Rails Downsides N1 risk
48. Tropical on Rails Downsides N1 risk More moving parts
49. Tropical on Rails Downsides N1 risk More moving parts Not true inheritance
50. Tropical on Rails Maybe’s Accounts Example
52. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types.
53. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types. Use Single-Table Inheritance
54. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types. Use Single-Table Inheritance If attributes differ significantly across types, and queries mostly happen within one concrete class.
55. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types. Use Single-Table Inheritance If attributes differ significantly across types, and queries mostly happen within one concrete class. Use Purely Abstract Classes
56. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types. Use Single-Table Inheritance If attributes differ significantly across types, and queries mostly happen within one concrete class. Use Purely Abstract Classes If attributes and validations differ, and you often need to query across all types.
57. If most attributes are shared, the set of subclasses is small and stable, and you often query across all types. Use Single-Table Inheritance If attributes differ significantly across types, and queries mostly happen within one concrete class. Use Purely Abstract Classes If attributes and validations differ, and you often need to query across all types. Use Delegated Types
58. THANK YOU!