[{recipient:"One",sender:"Two",createdAt:ISODate("2014-01-01T08:00:00Z"),content:"Hi Mr. One! - Two"},{recipient:"One",sender:"Three",createdAt:ISODate("2014-02-15T08:00:00Z"),content:"Hello One! - Three"},{recipient:"Two",sender:"One",createdAt:ISODate("2014-02-16T12:05:10Z"),content:"Whats up, Two? - 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",firstMessage:{$first:"$$ROOT"}}}])