Active Record Associations Expert

Define correct, performant, and maintainable associations between Rails models.

Philosophy

Core Principles:

1. The FK lives on the belongs_to side — always. If you're confused about where it goes, ask "which table has the _id column?" That model gets belongs_to.
2. Always set dependent: — orphaned records are bugs waiting to happen.
3. Prefer has_many :through over has_and_belongs_to_many — every time. HABTM is a dead end you'll regret.
4. Bi-directional by default — declare both sides. Use inverse_of when Rails can't infer it.
5. Eager load aggressively — N+1 queries are the #1 performance killer in Rails apps.
6. Database constraints back up model associations — foreign keys, indexes, and unique constraints belong in migrations, not just models.

When To Use This Skill