db={"conversations":[{"_id":ObjectId("5e35f2c840713a43aeeeb3d9"),"members":[{"uID":"1","j":1580580922},{"uID":"4","j":1580580922},{"uID":"5","j":1580580922}]}],"messages":[{"_id":ObjectId("5e35ee5f40713a43aeeeb1c5"),"c_ID":ObjectId("5e35f2c840713a43aeeeb3d9"),"fromID":"1","msg":"What's up?","t":111,"d":{"4":1580592039},"r":{"4":1580592339}},{"_id":ObjectId("5e35ee5f40713a43aeeeb1c6"),"c_ID":ObjectId("5e35f2c840713a43aeeeb3d9"),"fromID":"1","msg":"Hi","t":222,"d":{"4":1580592039},"r":{"4":1580592339}}]}

db.conversations.aggregate([{$match:{"members.uID":"1"}},{$lookup:{foreignField:"c_ID",from:"messages",localField:"_id",as:"messages"}},{"$unwind":"$messages"},{"$sort":{"messages.t":-1}},{"$group":{"_id":"$_id","lastMessage":{"$first":"$messages"},"allFields":{"$first":"$$ROOT"}}},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$allFields",{"lastMessage":"$lastMessage"}]}}}])