Since there’s no support in databases to point to entities outside the current database (well, even a different database on the same machine is not supported, so forget about a database on a different machine), the concept of foreign keys goes for a toss as well. Suddenly, the database becomes “dumb,” and data integrity is your problem.
But I haven’t found other sources regarding this topic, not even on the official documentation.
From the 6.0 document (upcoming version of MongoDB).
Starting in MongoDB 5.1, $lookup works across sharded collections.
So as long as your related data are in the same database (which makes sense), you can use $lookup to join them together in a query and manipulate the data as you please in your aggregation pipeline.
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