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$lookup Exercise in MongoDB | An Intro

In MongoDB, the $lookup stage acts as an aggregation pipeline operator. In other words, it offers a gateway to carry out left outer joins between two collections.

Furthermore, in MongoDB’s Aggregation Framework, this operator offers developers a flexible and strong toolset for processing and transforming data.

If you are still wondering why $lookup, consider a scenario where our data is dispersed across multiple collections, and we want to combine documents based on a shared field or condition. This is exactly where the $lookup comes in handy.

Understanding the Syntax

Let’s dive into the syntax of the $lookup stage:

• from: Specifies the target collection for the join.
• localField: Specifies the field from the input documents serving as the local join field.
• foreignField: Specifies the field from the target collection documents acting as the foreign join field.
• as: Specifies the name of the new array field cradling the joined documents.

Here is an example of how to put $lookup in action:

Suppose we have two collections, ‘orders’ and ‘products,’ and we want to get a list of orders with product details.

Here, the ‘orders’ collection has a field named ‘product_id,’ aligning with the ‘_id’ field in the ‘products’ collection. The $lookup stage combines documents from these collections according to certain fields.

As a result, we get a new array field orderDetails in each document in the orders collection. It contains the matched documents from the products collection.

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Conclusion

Today, our Support Engineers demonstrated how the $lookup stage in MongoDB is not just an operator. It is a key to unlocking the potential of aggregating and combining data from disparate collections.