You can use the `$lookup` aggregation stage to join two collections in MongoDB. This operation allows you to combine two collections into a single collection.
Here is an example:
Let’s assume `orders` collection is as follows:
```
{ “_id”: 1, “product”: “abc”, “quantity”: 10, “customer_id”: 1 }
{ “_id”: 2, “product”: “jkl”, “quantity”: 20, “customer_id”: 2 }
{ “_id”: 3, “product”: “xyz”, “quantity”: 30, “customer_id”: 1 }
```
And `customers` collection is as follows:
```
{ “_id”: 1, “name”: “John Doe” }
{ “_id”: 2, “name”: “Jane Doe” }
```
You can use the `$lookup` stage to join the `orders` collection with the `customers` collection on the `customer_id` field:
```
db.orders.aggregate([
{
$lookup:
{
from: “customers”,
localField: “customer_id”,
foreignField: “_id”,
as: “customer_details“
}
}
])
```
The output documents will contain a new array field called `customer_details` that contains the matching documents from the `customers` collection.
Remember that MongoDB isn’t a relational database, so heavy use of `$lookup` can be a sign that your data might be better structured in a different type of database.
