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Combining Collections

This guide demonstrates how to merge data from two collections using MongoDB's aggregation framework.

Use Case

You need to combine related data from separate collections - in this case, retrieving listings along with their associated reviews.

Prerequisites

This guide uses the collections we created in the previous guide where we split the listingsAndReviews collection into separate listings and reviews collections.

Using $lookup

The $lookup operation performs a left outer join, allowing you to incorporate documents from one collection into matching documents from another.

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from monggregate import Pipeline

# Define the pipeline
pipeline = Pipeline()
pipeline.lookup(
    right="reviews",         # Target collection we're joining with
    left_on="_id",           # Field in the listings collection
    right_on="listing_id",   # Field in the reviews collection that matches
    name="reviews"           # Output field to store the joined data
)

# Execute the pipeline on the listings collection
results = db["listings"].aggregate(pipeline=pipeline.export())

Result Structure

After execution, each document from the listings collection will contain a new reviews array field with all matching review documents.

Additional Options

For more control over joined data: - Use pipeline.project() after lookup to shape the output - Add matching conditions with let and pipeline parameters - Explore the $unwind stage to process array results