[{recipient:"One",sender:"Two",createdAt:ISODate("2014-01-01T08:00:00Z"),message:"High Two!"},{recipient:"One",sender:"Three",createdAt:ISODate("2014-02-15T08:00:00Z"),message:"Hello Three!"},{recipient:"Two",sender:"One",createdAt:ISODate("2014-02-16T12:05:10Z"),message:"Whats up, One?"}]

db.collection.aggregate([{$match:{$and:[{$or:[{recipient:"One"},{sender:"One"}],},]}},{$addFields:{conversationWith:{$cond:{if:{$eq:["$sender","One"]},then:"$recipient",else:"$sender"}}}},{$sort:{createdAt:-1}},{$group:{_id:"$conversationWith",message:{$first:"$$ROOT"}}}])