[{"_id":"gQnYi2e54zADwgipx","users":["aFHJBrKu54y5mWjY3","CnugxoBWs4ox6vG2k"],"lastMessageAt":ISODate("2020-06-22T22:50:35.579Z"),"messages":[{"id":"4d2219d05645a3991d3aae89","addedAt":ISODate("2020-06-22T22:50:35.579Z"),"userId":"CnugxoBWs4ox6vG2k","newFor":["aFHJBrKu54y5mWjY3","CnugxoBWs4ox6vG2k"],"message":"Test"},{"id":"b42641118bb080cb9122062f","addedAt":ISODate("2020-06-22T22:48:24.359Z"),"userId":"aFHJBrKu54y5mWjY3","newFor":["aFHJBrKu54y5mWjY3"],"message":"Test 2"},{"id":"244e77bb8324dc0b0f0e2def","addedAt":ISODate("2020-06-22T22:48:14.643Z"),"userId":"CnugxoBWs4ox6vG2k","newFor":["aFHJBrKu54y5mWjY3"],"message":"Test 3"}]}]

db.collection.aggregate([{"$project":{"messages":{"$map":{"input":"$messages","as":"m","in":{"id":"$$m.id","addedAt":"$$m.addedAt","userId":"$$m.userId","message":"$$m.message","newFor":{$filter:{"input":"$$m.newFor","as":"n","cond":{"$ne":["$$n","aFHJBrKu54y5mWjY3"]}}}}}}}}])