How to perform a JOIN in MongoDB

For a full overview of MongoDB and all my posts on it, check out my overview.

If you have relational data in your MongoDB instance, you can perform an operation similar to a JOIN typically done in SQL queries.

Consider the following data set:

db.podcasts.insertMany([
	{id: 1, name: "Off The Clock", category: "Technology", rss: "https://anchor.fm/s/76aafa5c/podcast/rss"},
	{id: 2, name: "Tech Over Tea", category: "Technology", rss: "https://anchor.fm/s/149fd51c/podcast/rss"}
]);

db.episodes.insertMany([
	{podcast_id: 1, title: "Resume Tips", published_on: "2022-01-11"},
	{podcast_id: 2, title: "#75 Welcome Our Hacker Neko Waifu | Cyan Nyan", published_on: "2021-08-04"},
	{podcast_id: 2, title: "Square Enix Refuses To Take My Money | Solo", published_on: "2022-01-26"},
	{podcast_id: 1, title: "Find the Right Job", published_on: "2022-01-25"}
]);

If you want to get every podcast with all of its associated episodes, you can accomplish this using the $lookup aggregation stage.

The aggregation would look something like this:

db.podcasts.aggregate([
	{ $lookup: {
		from: "episodes", // Name of the other collection to "join" from
		localField: "id", // Name of the field your current documents contain to compare with
		foreignField: "podcast_id", // Name of field to compare to in the "from" collection's documents
		as: "episodes" // What to call the field that contains the array of sub documents that matched
	}}
]);

Any documents in the episodes collection that had a podcast_id that matched one of the podcasts documents would have been added to an array of sub-documents called episodes.

The result would look like this:

[
	{
		id: 1,
	 	name: "Off The Clock",
		category: "Technology",
		rss: "https://anchor.fm/s/76aafa5c/podcast/rss",
		episodes: [
			{podcast_id: 1, title: "Resume Tips", published_on: "2022-01-11"},
			{podcast_id: 1, title: "Find the Right Job", published_on: "2022-01-25"}
		]
	},
	{
		id: 2,
		name: "Tech Over Tea",
		category: "Technology",
		rss: "https://anchor.fm/s/149fd51c/podcast/rss",
		episodes: [
			{podcast_id: 2, title: "#75 Welcome Our Hacker Neko Waifu | Cyan Nyan", published_on: "2021-08-04"},
			{podcast_id: 2, title: "Square Enix Refuses To Take My Money | Solo", published_on: "2022-01-26"},
		]
	}
]

This would be the equivalent of the SQL query:

SELECT
	*
FROM
	podcasts
LEFT JOIN episodes ON episodes.podcast_id = podcasts.id;

I specified LEFT JOIN in the SQL example because if no documents match the $lookup aggregation stage, the podcasts would still be returned with the episodes field being an empty array.

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