$lookup on ObjectId"s in an array

📝 Title: Unraveling the Mystery of $lookup on ObjectId's in an Array

🔍 Introduction: Are you grappling with the syntax for performing a $lookup on an array of ObjectIds? Fear not! In this blog post, we will tackle this question head-on and provide you with easy solutions to address this common issue. By the end of this post, you'll be equipped with the knowledge to effortlessly perform $lookup queries on ObjectId arrays. Let's dive in! 💪

💡Problem: The challenge lies in executing a $lookup on a field that contains an array of ObjectIds, rather than just a single ObjectId. As illustrated in the example below, we want to fetch additional details from the "products" collection based on the ObjectIds stored in the "products" array within the "orders" document.

💻 Example Order Document:

{
  _id: ObjectId("..."),
  products: [
    ObjectId("..<Car ObjectId>.."),
    ObjectId("..<Bike ObjectId>..")
  ]
}

⛔ Not Working Query: Let's take a look at the query that doesn't yield the desired result:

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

🚀 Solution: To successfully accomplish the $lookup on an ObjectId array, we need to modify our query slightly. In the "localField" parameter, we need to use the "$in" operator in combination with the ObjectId array. The updated query is as follows:

db.orders.aggregate([
    {
       $lookup:
         {
           from: "products",
           localField: "products",
           foreignField: "_id",
           as: "productsObjects"
         }
    },
    {
       $addFields:
         {
           "productObjects":
             {
               $filter:
                 {
                   input: "$productObjects",
                   as: "product",
                   cond: { $in: ["$$product._id", "$products"] }
                 }
             }
         }
    }
])

Voila! With this modification, we can now achieve the desired result:

✨ Desired Result:

{
  _id: ObjectId("..."),
  products: [
    ObjectId("..<Car ObjectId>.."),
    ObjectId("..<Bike ObjectId>..")
  ],
  productObjects: [
    {<Car Object>},
    {<Bike Object>}
  ],
}

📣 Call-to-Action: Now that you've mastered the art of $lookup on ObjectId's in an array, why not put your newfound knowledge into practice? Experiment with different scenarios and share your successes or challenges in the comments section below. Let's learn and grow together! 🌟

That wraps up our guide on performing $lookup on ObjectId's in an array. We hope this article has enlightened you and made your MongoDB journey smoother. Stay tuned for more insightful content from our blog. Happy lookup-ing! 🙌