Ecto Essentials

Use this skill before modifying ANY schema, query, or migration.

Canonical FP bar: docs/fcis-engineering-rules.md — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Build changesets/Multi in pure-ish functions; run Repo once at the context edge.

RULES — Follow these with no exceptions

1. Add database constraints (unique_index, foreign_key, check_constraint) AND changeset validations — both layers are required 2. Add indexes on foreign keys and frequently queried fields — never omit indexes on foreign keys 3. Parameterize all user input in queries — never interpolate values into SQL fragments, always use ^ 4. Never combine schema changes and data backfill in the same migration

FCIS at this boundary

Ecto is the persistence edge. Prefer pure functions that build changesets/queries/Multi; execute with Repo once in the context shell.