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; let Ecto manage the relationship
2. Use Ecto.Multi for operations spanning multiple unrelated tables — not nested changesets; Multi provides explicit rollback control
3. Set on_delete explicitly in migrations — :delete_all for owned children, :nothing for references to independent entities
4. Always create indexes on foreign key columns — missing FK indexes cause slow joins and lookups on the child table
5. Use on_replace: :delete in cast_assoc for list management — allows removing items by omitting them from the input
6. Preload associations before updating them — cast_assoc/3 raises (attempting to cast or change association ... that was not loaded) if the association isn't preloaded; preload before update

cast_assoc for Nested Creates

Create parent and children in a single operation. Ecto sets foreign keys automatically.