mongodb-aggregation

I have mongo query which does the group operation on the documents. I have almost got the expected results except that I want to refine the results without empty or null values. Currently my query looks like this: db.productMetadata.aggregate([{$group:{"_id":{"color":"$productAttribute.colour","gender":"$productAttribute.gender"},"count" : {$sum : 1}}}]); And the results looks something like this: { "_id" : { "color" : "BLUE", "gender" : "MEN" }, "count" : 1 } { "_id" : { }, "count" : 4 } { "_id" : { "color" : "NA", "gender" : "WOMEN" }, "count" : 1 } { "_id" : { "color" : "BLACK", "gender" :

I'm trying to do something like this: use user; db.user.aggregate([ { $lookup: { from: "organization.organization", localField: "organizationId", foreignField: "uuid", as: "user_org" } } ]) user and organization are in two different databases. If this is not possible, what are the alternatives? Clement Amarnath Is it possible to do a $lookup aggregation between two databases in Mongodb? It is not possible to query using lookup in two different db's. $lookup in mongodb supports Performs a left outer join to an unsharded collection in the same database. { $lookup: { from: <collection to join>,