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

db.collection.aggregate([{$match:{$and:[{$or:[{recepient:"One"},{sender:"One"}],},]}},{$addFields:{conversation:{$cond:{if:{$references:{$exists:false}},then:"$_id",else:{$arrayElemAt:["$references",0]}}}}},{$sort:{createdAt:-1}},{$group:{_id:"$conversation",message:{$first:"$$ROOT"}}}])