# Master-detail, lookup and junction object relationships

*Platform Administrator II Study Guide · Objects and Applications · Last updated October 26, 2026 · https://www.certifyforce.com/study-guide/platform-administrator-ii/objects-and-applications/master-detail-lookup-and-junction-object-relationships*

How to choose between master-detail, lookup, hierarchical and junction designs from ownership, sharing, deletion, roll-up and data-volume requirements, how to convert a lookup that already holds data, and what blocks deleting an object that sits in a relationship.

- Certification: Salesforce Platform Administrator II
- Exam section: Objects and Applications
- Study guide: https://www.certifyforce.com/study-guide/platform-administrator-ii

**Official exam objective:**

> Given a scenario, determine the appropriate solution to enhance or extend objects (master-detail, lookup, junction object, related list, record type, Schema Builder, and Object Creator).

## What the exam expects

- Choose master-detail, lookup, two lookups, a junction or a hierarchical relationship from the requirements.
- Add master-detail to an object that holds records; predict what conversion does to sharing and reports.
- Roll up and report across multilevel master-detail relationships.
- Build a junction object: primary master, related lists, icons, roll-ups.
- Diagnose blocked object deletion and missing field history.

Key terms:
- **Master-detail:** the master controls the detail's owner, security and deletion.
- **Lookup:** a looser link; the child keeps its own owner and sharing.
- **Junction object:** a custom object with two master-detail relationships (many-to-many).
- **Primary master:** the junction's first master-detail relationship.
- **Hierarchical relationship:** a User-only lookup that can't loop back to the same user.

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Relationships per custom object | Up to 2 master-detail and up to 40 total | [Object Relationships Overview](https://help.salesforce.com/s/articleView?id=overview_of_custom_object_relationships.htm&type=5) |
| Child volume best practice | Don't exceed 10,000 child records for a master-detail relationship | [Object Relationships Overview](https://help.salesforce.com/s/articleView?id=overview_of_custom_object_relationships.htm&type=5) |
| Object already has data | Create a lookup, fill every record, convert to master-detail | [Considerations for Object Relationships](https://help.salesforce.com/s/articleView?id=platform.relationships_considerations.htm&type=5) |
| Master-detail to lookup | Blocked while the master has roll-up summary fields | [Considerations for Object Relationships](https://help.salesforce.com/s/articleView?id=platform.relationships_considerations.htm&type=5) |
| Delete this record also | Cascade-delete, disabled by default; contact Salesforce to enable | [Object Relationships Overview](https://help.salesforce.com/s/articleView?id=overview_of_custom_object_relationships.htm&type=5) |
| Junction primary master | First master-detail created; sets look and feel, Owner value and division | [Create a Many-to-Many Object Relationship](https://help.salesforce.com/s/articleView?id=sf.relationships_manytomany.htm&type=5) |
| Junction roll-ups | Can be created on both masters | [Considerations for Object Relationships](https://help.salesforce.com/s/articleView?id=platform.relationships_considerations.htm&type=5) |
| Master-side custom object | Can't be deleted; deleting the detail side converts the field to a lookup | [Considerations for Object Relationships](https://help.salesforce.com/s/articleView?id=platform.relationships_considerations.htm&type=5) |

## Check yourself

- Why can't detail-side custom objects use queues or manual sharing?
- When can a lookup be converted to master-detail?
- Why might custom reports stop working after a relationship conversion?
- What does field history on a child show when its lookup parent is deleted?

## Sources

- [Considerations for Object Relationships](https://help.salesforce.com/s/articleView?id=platform.relationships_considerations.htm&type=5)
- [Object Relationships Overview](https://help.salesforce.com/s/articleView?id=overview_of_custom_object_relationships.htm&type=5)
- [Create a Many-to-Many Object Relationship](https://help.salesforce.com/s/articleView?id=sf.relationships_manytomany.htm&type=5)
- [Roll-Up Summary Field](https://help.salesforce.com/s/articleView?id=platform.fields_about_roll_up_summary_fields.htm&type=5)
- [Create a Roll-Up Summary Field](https://help.salesforce.com/s/articleView?id=platform.fields_defining_summary_fields.htm&type=5)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 11 linked practice questions, and unlimited timed mock exams are included with the Platform Administrator II pack (USD $19): https://www.certifyforce.com/practice-exams