$lookup MongoDB cross database query

[dahaol hao]

Is it possible to do a $lookup aggregation between two databases in Mongodb?

For example:

I have a collection, called "bookshelves" in a database(just call it aaa). But have another collection, called "picture_books" in another database (just call it bbb).

I want to do like this:

db.bookshelves.aggregate([{$match: {"child_id":262}},{$lookup: {from: "picture_books", localField: "bookshelfable_id", foreignField: "book_id", as: "picture_books"}}]).pretty()

it doesn't work. 

Mongodb says

"Performs a left outer join to an unsharded collection in the same database to filter in documents from the “joined” collection for processing."

so, How can i solve it?

[Asya Kamsky]

Make sure both collections are in the same database.  Cross-database $lookup is not allowed.

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Asya Kamsky
Lead Product Manager
MongoDB
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