db={"col1":[{_id:1,name:"Product1",units:20},{_id:2,name:"Product2",units:10},{_id:3,name:"Product3",units:50},{_id:4,name:"Product4",units:4}],"col2":[{_id2:1,inventory:1,quantity:20},{_id2:2,inventory:1,quantity:10},{_id2:3,inventory:1,quantity:50},{_id2:4,inventory:2,quantity:4},{_id2:5,inventory:2,quantity:45},{_id2:6,inventory:3,quantity:49}]}

db.col1.aggregate([{$lookup:{from:"col2",let:{inventory:"$_id"},pipeline:[{$match:{$expr:{$eq:["$$inventory","$inventory"]}}},{$group:{_id:null,quantity:{$sum:"$quantity"}}}],as:"inventoryList"}}])