Yes Mongo supports joins. But, I wouldn’t use them. Application servers scale much easier than database servers. You’re not getting any efficiency gains from doing server side joins over just reading documents from the left side and doing an “in” query with the ids from the right side. Assuming you are doing the equivalent of a left outer join.

In fact, if you are using C#. You could use the same LINQ syntax either way.

The OP just said it would be hard to do that query (regardless of performance) and I'm saying it's totally capable.