Creating the junction object takes a few minutes; the design decisions (custom fields, primary master, related list configuration) matter more than the click path.
- Identify the two parent objects
Confirm the use case requires many-to-many. A one-to-many relationship (Account-to-Contact via Contact.AccountId) does not need a junction.
- Create the custom junction object
Setup, Object Manager, New Custom Object. Name it descriptively (Student_Course, Contact_Campaign, Account_Partner).
- Add the two master-detail relationships
On the junction, add Master-Detail to parent A, then Master-Detail to parent B. The first one is the Primary Master.
- Add custom fields if needed
Role, Date Range, Status, Percentage. Whatever attributes the relationship needs.
- Configure page layouts on the parents
Add a Related List for the junction on each parent''s page layout. Users see and create junction records from the parent record.
- Verify with sample data
Create test records on both parents and on the junction. Verify the related lists, the SOQL queries, and the sharing all work as expected.
The custom object that holds the relationships.
The Primary Master relationship.
The Secondary Master relationship.
Relationship attributes.
Required to show the related lists.
- Primary vs. Secondary Master matters. The Primary controls sharing and ownership; pick deliberately at junction creation time.
- Standard junction objects have special behavior. CampaignMember, OpportunityContactRole, AccountContactRelation cannot be replaced by custom junctions without losing platform features.
- Junction rows are full custom-object records. High-volume many-to-many relationships consume storage and query limits.
- Cascading deletes from either parent remove the junction row. Plan for this behavior; soft deletes via custom flag work better in some scenarios.