mongodbHow to implement one-to-many relation in MongoDB?

MongoDB supports one-to-many relationships using the $lookup operator. This operator allows you to join two collections together and return the matching documents from both collections.

For example, the following code will join the orders collection with the products collection and return the matching documents:

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

The code above will return the following output:

{
    "_id" : ObjectId("5f3d7f9f8f9f9f9f9f9f9f9f"),
    "productId" : ObjectId("5f3d7f9f8f9f9f9f9f9f9f9f"),
    "products" : [
        {
            "_id" : ObjectId("5f3d7f9f8f9f9f9f9f9f9f9f"),
            "name" : "Product 1",
            "price" : 10
        }
    ]
}

Code explanation

1. $lookup - This is the operator used to join two collections.
2. from - This is the collection to join with.
3. localField - This is the field in the current collection to match with the foreignField.
4. foreignField - This is the field in the foreign collection to match with the localField.
5. as - This is the name of the array field that will contain the matching documents from the foreign collection.

Helpful links

• MongoDB Documentation - $lookup

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