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Many-to-many with junction objects

A Student takes many Courses and a Course has many Students. Relational databases solve this with a join table, and Salesforce does exactly the same thing, calling it a junction object.

Contact (student)  ◄──M-D──  Enrollment__c  ──M-D──►  Course__c

Enrollment__c has two master-detail fields:

Contact__c → Contact
Course__c → Course__c

Each Enrollment record links one student to one course, and it can hold its own data: Status__c, Grade__c, Enrolled_On__c.

Why master-detail on both sides?

Deleting a Course cleans up its Enrollments automatically
You get roll-ups on both parents: "number of students" on Course, "courses taken" on Contact
Sharing is inherited, so nobody can see an Enrollment without access to its parents

The primary master

The first master-detail field you create is the primary relationship. It decides:

Who "owns" the junction record for sharing purposes
The look and feel (tab style) of the junction object

Querying it

// all courses a student is enrolled in
SELECT Course__r.Name, Status__c
FROM Enrollment__c
WHERE Contact__c = :studentId

You'll build exactly this in the boss fight at the end of this chapter.

Check your understanding

1.How do you model a many-to-many relationship between Students and Courses?

A multi-select picklist on StudentA junction object (e.g. Enrollment__c) with two master-detail fields, one to Student and one to CourseA lookup from Course to StudentIt isn't possible in Salesforce

2.On a junction object with two master-detail relationships, which master controls ownership and sharing?

The primary master (the first master-detail field you created)Whichever was edited most recentlyNeither. Junction records have their own ownerBoth equally