Question

Suppose I have a mongodb collections A and B.
While A has a foreign key to B called b_id.

How do I do in MongoDB (or even in ruby is better) the following query:

select * from A where b_id not in (select id from B where <some_condition> );

Answer 1

MongoDB doesn’t do relational queries (it’s not called NoSQL without cause) — but there are other ways you can accomplish this.

1. Run a two-step query in your Mongo client, first getting the inner array, then using the $nin operator:

   var ids = db.B.find( conditions );
   db.A.find( { b_id: { $nin: ids } } );

2. If you’re still in the design phase you can consider using nested documents for your schemas. For example, if B has many A, then you could add an array of A to each B as a property. (Keep in mind, though, that there is a limit to document size in MongoDB, so this is not a good solution if the number of A in any given B could be very large.)