MongoDB $lookup

Joins in MongoDB are performed using $lookup.

Example

{
  "$lookup": {
    "from": "orders",
    "localField": "_id",
    "foreignField": "customerId",
    "as": "orders"
  }
}

This attaches all orders for each customer.

Tips

• $lookup can be expensive on large collections
• Use indexes on foreignField
• Consider $unwind to flatten arrays

MongoDB is not a relational database—use joins sparingly.