What is a junction object in Salesforce?

Quick definition

A junction object in Salesforce is a custom object used to model a many-to-many relationship between two other objects. It acts as an intersection (or join) table by holding two master-detail relationships—one to each parent object—so a single record on one object can be associated with multiple records on the other and vice versa.

When to use a junction object

Use a junction object when you need to represent a many-to-many relationship. Common examples include:

Students and Courses — a Student can enroll in many Courses and a Course can have many Students (junction: Enrollment).
Products and Orders — an Order can include many Products and a Product can be part of many Orders (junction: OrderLineItem).

How it works (structure)

Technically, a junction object is a custom object with two master-detail relationships (to Object A and Object B). Each junction record contains the combined linkage and any attributes specific to the relationship (for example, Enrollment might store EnrollmentDate, Grade, or Status).

Key benefits

Using a junction object provides:

True many-to-many relationships in the Salesforce data model.
Ability to add fields on the relationship itself (e.g., enrollment date, role, quantity).
Roll-up summary fields on the master objects (because master-detail supports roll-ups).

Security and ownership

When using two master-detail relationships, the junction object inherits security and sharing from the primary master. In practice, the junction record’s owner is determined by the master-detail configuration and the record is controlled by the parent(s). This is useful when relationship records should not exist independently without parents.

Alternative: Two lookups

If you require independent sharing, separate ownership, or the child record to survive when a parent is deleted, use two lookup relationships instead of master-detail. This sacrifices native roll-up summary capability, but gives more flexible sharing and deletion behavior.

Examples: SOQL and Apex

Assume a junction object named Enrollment__c with master-detail relationships to Student__c and Course__c named Student__r and Course__r.

Query all courses for a student:

List enrollments = [SELECT Id, Course__r.Name, Course__c FROM Enrollment__c WHERE Student__c = :studentId];

Query students on a course using a parent-to-child subquery:

Course__c c = [SELECT Id, Name, (SELECT Id, Student__r.Name FROM Enrollments__r) FROM Course__c WHERE Id = :courseId];

Best practices

Name the junction object clearly (e.g., Enrollment, OrderItem) and include relationship fields with clear API names.
Use master-detail if you need roll-up summaries and strict ownership; otherwise use lookups for flexible sharing.
Validate data with triggers or flows if business rules apply to the relationship attributes.
Avoid unnecessary junctions — model only when many-to-many is required.