MongoDB — Relationships

Relationship Types

Type	Description
One-to-One	Single related document
One-to-Many	Multiple related documents
Many-to-Many	Multiple on both sides

Embedding (One-to-One)

{
  name: "John",
  profile: {
    bio: "Developer",
    website: "example.com"
  }
}

Referencing (One-to-Many)

// User document
{ _id: ObjectId("..."), name: "John" }

// Order documents
{ userId: ObjectId("..."), total: 100 }

Population

db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  }
])

Mini Practice

1. Design one-to-one
2. Design one-to-many
3. Use $lookup for references
4. Choose embedding vs referencing

Up Next

Continue with Transactions — multi-document transactions.