Extending Custom Objects and Applications
Schema building blocks
Know the relationship family: lookup (loose, optional, no cascade delete, no roll-up), master-detail (tight, required, cascade delete, determines the child owner, supports roll-up summaries, max two per custom object), junction objects for many-to-many, hierarchical (user-to-user only), external lookup (parent matched by an external ID) and indirect lookup (parent matched through another object's external ID). Roll-up summaries exist only on master-detail; cross-object formula fields can climb a limited number of parent hops but can never read child records directly.
Also own record types and page layouts, dynamic forms on Lightning record pages, controlling and dependent picklists, validation rules, custom apps and tabs, and the choice between custom settings (configuration data, SOQL-accessible, data not deployed) and custom metadata types (deployable records, preferred for configuration that must move between orgs).
Traps
- Allow Reparenting must be enabled before a master-detail child can change parents; lookups always allow it.
- You cannot promote a lookup to master-detail if child records would be orphaned, and field type changes on populated fields are restricted.
- Lookup skew - huge numbers of children on one parent - degrades performance; consider a junction object or an indexed external ID.
- Formula fields cannot be marked required at the database level; use a validation rule instead.
- Deleting a master-detail parent deletes its children; a lookup merely blanks or blocks the delete.
Worked example
Invoices need totals from line items and must disappear with their lines. That requires master-detail (roll-up summaries), not a lookup. If the lines must survive the invoice, use a lookup plus automation that re-parents and recalculates.