Many-to-Many Relationships

Introduction

Many-to-many (M:N) is solved in MongoDB by storing an array of references on one or both sides, or by introducing a junction collection — mirroring the SQL join table when the relationship has its own attributes (timestamps, role, etc).

Syntax reference

// Embed ids on one side
{ user: "Ada", teamIds: [ObjectId("..."), ObjectId("...")] }

// Junction collection (when relationship has attributes)
// memberships: { userId, teamId, role, joinedAt }

Real-world use

Slack uses a junction collection for user ↔ workspace membership, storing role and join date on the edge.

Best practices

• Embed ids when the relationship has no attributes.
• Use a junction collection when it does.