General data access concepts

General data access interviews judge pattern literacy before framework trivia. Topics: Active Record versus Data Mapper, repository boundaries, Unit of Work, query builders, migrations, lazy versus eager loading, N+1, and ORM transactions.

On interviews: walk a use case and label where objects live, when SQL runs, and what invariant a transaction protects.

Common pitfalls: pattern slogans without trade-offs; fixing performance only by switching ORM; migrations as an afterthought.

The trade-off is choosing the lightest pattern that still keeps tests and deploys safe.

Checklist:

• Map pattern to coupling and test seams.
• Measure query count before optimizing tools.
• Review migration SQL like application code.
• Keep transactions scoped to invariants.