I know a way to join multiple collections using a same key, and I thinks
its not possible to join multiple collections on multiple keys but
wondering anyone know a way to do it.

Lets say I have a collection AAA with fields A, B, and C, and I have a
collection called BBB with fields B, D and E, and I have another collection
called CCC with fields C, F, G.

In collection AAA field B is DB Ref for field B in collection BBB, and
field C in collection AAA is a DB ref for field C in collection CCC. Now I
want join these three collections like AAA join BBB on AAA.B =BBB.B , BBB
join CCC on BBB.C = CCC.C.

I know that I can embed one collection in another but I have a situation
where I can't embed everything but need to do a adhoc report across
multiple collection. So how do I do it?

Hi Anand,

MongoDB doesnt support joins natively, you will need to perform the joins
within your application.

Cheers,

David

Anand -- are you trying to do this with standard queries, or with mapReduce?

 -William