MongoDB does not support joins; If you need to retrieve data from more than one collection you must do more than one query ... you can generally redesign your schema ... you can de-normalize your data easily.
This is a much larger issue than it seems - nested collections aren't first class objects in MongoDB - the $ operator for querying into arrays only goes one level deep, amongst its other issues, meaning that often-times you must break things out into separate collections. This doesn't work either, though, as there are no cross-collection transactions, so if you need to break things into separate collections, you can't guarantee a write to each collection will go through properly. (Though, I suppose if you're using the latest version, you could lock your whole database)
Also, the postgres integration (linked/discussed) here on HN