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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