Ecto Changeset Patterns

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. Define separate named changesets per operation — registration_changeset, email_changeset, password_changeset, etc. 2. Never require foreign key fields in cast_assoc child changesets — cast_assoc sets them automatically 3. Compose changesets with pipes — each validation step is a separate, reusable function 4. Always pair unsafe_validate_unique with unique_constraint — fast UI feedback plus race-safe enforcement 5. Use update_change/3 for field transformations — trim, downcase, and slugify inside the changeset 6. Accept opts \\ [] for conditional validation — toggle hashing or uniqueness checks per call site 7. Validate at the changeset level, not in context functions — keep validation next to the schema

FCIS at this boundary

Changeset functions should be pure data transformers (attrs in → changeset out). Persist with Repo in the context, not inside the schema module beyond optional helpers.