Many-to-Many

Many-to-many (n:m) relationships describes a scenario where multiple records from a table can relate to multiple records in another table. An example of this would be the relationship between students and courses, where a single student can enroll in multiple courses, and a single course can have multiple students.

The Serverpod framework supports these complex relationships by explicitly creating a separate model, often called a junction or bridge table, that records the relation.

Overview

In the context of many-to-many relationships, neither table contains a direct reference to the other. Instead, a separate table holds the foreign keys of both tables. This setup allows for a flexible and normalized approach to represent n:m relationships.

Modeling the relationship between Student and Course, we would create an Enrollment model as a junction table to store the relationship explicitly.

Defining the Relationship

In the following examples we show how to configure a n:m relationship between Student and Course.

Many tables

Both the Course and Student tables have a direct relationship with the Enrollment table but no direct relationship with each other.

# course.yaml
class: Course
table: course
fields:
  name: String
  enrollments: List<Enrollment>?, relation(name=course_enrollments)

# student.yaml
class: Student
table: student
fields:
  name: String
  enrollments: List<Enrollment>?, relation(name=student_enrollments)

Note that the name argument is different, course_enrollments and student_enrollments, for the many tables. This is because each row in the junction table holds a relation to both many tables, Course and Student.

Junction Table

The Enrollment table acts as the bridge between Course and Student. It contains foreign keys from both tables, representing the many-to-many relationship.

# enrollment.yaml
class: Enrollment
table: enrollment
fields:
  student: Student?, relation(name=student_enrollments)
  course: Course?, relation(name=course_enrollments)
indexes:
  enrollment_index_idx:
    fields: studentId, courseId
    unique: true

The unique index on the combination of studentId and courseId ensures that a student can only be enrolled in a particular course once. If omitted a student would be allowed to be enrolled in the same course multiple times.