Add a Lookup field on a child custom object pointing to a parent, then verify the parent's detail page shows the new related list and reporting picks up the join.
- Open the child object
Setup, Object Manager, find the child object (the one that will hold the relationship field). Click Fields and Relationships, New.
- Choose Lookup Relationship
On the field type selector, pick Lookup Relationship. Click Next. If you need strict ownership and roll-ups, choose Master-Detail Relationship instead.
- Pick the related object
Select the parent object from the dropdown. Standard objects (Account, Contact, Opportunity) appear alongside custom ones. Click Next.
- Set field label and name
Enter the field label as it will appear on the child layout. The API name defaults from the label; adjust if needed. Add a help text describing the relationship.
- Configure related list
Choose the columns to show on the parent's related list and the row count (typically 5 or 10). Save.
- Verify on the parent page
Open any parent record. The new related list appears on its detail page. Add a child record to confirm the relationship is wired correctly.
- Each object supports at most 2 master-detail relationships and 40 total relationships of all types. Plan the data model upfront; refactoring after hitting the limit is painful.
- Switching a Master-Detail to a Lookup decouples sharing inheritance and removes any roll-up summary fields that depended on it. The conversion is one-way without data loss but is not safely reversible.
- Roll-up summary fields are limited to 25 per object. Heavy use can hit the cap; consolidate by combining filters or moving aggregations to scheduled flows.
- Deleting a master record cascade-deletes every detail record. There is no undelete-with-children flow; recovery requires a full sandbox restore.