MongoDB: Subquery to Another DB in The Same Server

Last week, I needed to find articles by user in different DB on the same server. The query won’t be used in code, it is just for a quick check. So I decided to make a subquery between DBs.

The structure is like this:

- MongoDB Server
    - account DB
        - user Collection
            - _id
            - username
            - etc
    - content DB
        - article Collection
            - _id
            - user_id: refer to accounts.user._id
            - title
            - etc

After some googling and trial and error, I came up with this solution:

1. Connect to MongoDB server.
2. Run the query below:

   use account;
   
   db.getCollection('user')
       .find({ user_name: 'hassan' }, { _id: 1 })
       .forEach((user) => {
           db.getSiblingDB('content') // use content DB to query articles owned by a user
               .article.find({ user_id: user._id }, { _id: 1, title: 1 })
               .forEach((article) => {
                   var result = { user_id: user._id, article_id: article._id, title: article.title };
                   printjson(result);
               });
       });