Database
[{warehouseID:"1234",transactions:[{"qtyIn":"10","transDateTime":ISODate("2019-09-10T18:54:41.983Z")},{"qtyOut":"11","transDateTime":ISODate("2019-08-10T18:54:41.983Z")},{"qtyOut":"200","transDateTime":ISODate("2019-02-10T11:54:41.983Z")}],}]
Query
db.collection.aggregate([{$addFields:{transactions:{$filter:{input:"$transactions",cond:{$ne:["$$this.qtyIn",undefined]}}}}}])