Ecto Nested Associations

RULES — Follow these with no exceptions

1. Use cast_assoc/3 for has_many/has_one — never manually insert children in a separate step
2. Use Ecto.Multi for operations spanning multiple unrelated tables — do NOT use Ecto.Multi for nested associations
3. Set on_delete explicitly in migrations — :delete_all for owned children, :nothing for independent entities
4. Always create indexes on foreign key columns
5. Use on_replace: :delete in cast_assoc for list management — allows removing items by omitting them
6. Preload associations before updating them — cast_assoc compares against currently loaded data
7. Do NOT require foreign keys in child changesets — cast_assoc sets them automatically
8. Use Repo.transaction/1 with Ecto.Multi — wrap multi-table operations for atomicity

End-to-End Workflow

Follow this sequence for ANY nested association or multi-table operation: