[{"_id":1,"name":"a","platformA":{"name":"a"},"platformB":{"name":"b"}},{"_id":2,"platformA":{"name":"a"},"platformB":{"name":"b"}}]

db.collection.find({},{_id:1,name:1,platformB:1,platformA:{$cond:[{$in:["$_id",[1]]},0,"$platformA"]}})