db={"invoice":[{"UniqueID":NumberInt(3851963),"InvNum":NumberInt(35602582),"Location":"035","LINEITEM":NumberInt(1),"CUSTID":"001595","DEPTID":"035360","PRODID":"003600","ProdDesc":"MISC MERCHANDISE",}],"tax":[{"InvoiceDate":"7/15/2017 12:00:00 AM","Location":"035","InvNum":NumberInt(35602582),"LineItem":NumberInt(1),"UniqueID":NumberInt(1),"taxCode":"mu48","taxrate":NumberInt(2),},{"InvoiceDate":"7/15/2017 12:00:00 AM","Location":"035","InvNum":NumberInt(35602582),"LineItem":NumberInt(1),"UniqueID":NumberInt(1),"taxCode":"mu48","taxrate":NumberInt(2),}]}

db.invoice.aggregate([{"$lookup":{"from":"tax","let":{"invNum":"$InvNum","lINEITEM":"$LINEITEM"},"pipeline":[{"$match":{"$expr":{"$and":[{"$eq":["$InvNum","$$invNum"]},{"$eq":["$LineItem","$$lINEITEM"]}]}}}],"as":"clinics"}}])