Please see http://stackoverflow.com/questions/35698842/join-operation-not-working-mongo-c-sharp-driver-linq#comment60869673_35698842

When I try to perform join operation I am getting joint document collection count as zero. It happens when we link (objectId) _id fields both the side, its works when you link up some other datatype likes strings, it could be bug in the driver which is not accepting objectId.