db={"groups":[{"_id":ObjectId("602ed22af42c404096407dda"),"groupName":"Name"}],"inventory":[{"_id":ObjectId("602ed22af42c404096407ddc"),"linkedGroup":ObjectId("602ed22af42c404096407dda"),"list":[{"_id":ObjectId("602eeb0621a11045638b7082"),"data":{"number":2},},{"_id":ObjectId("602eec75c37147459ed7b12c"),"data":{"number":5}},{"_id":ObjectId("602eeb0621a11045638b7082"),"data":{"number":30},},{"_id":ObjectId("602eec75c37147459ed7b12c"),"data":{"number":4}}]}]}

db.groups.aggregate([{"$lookup":{"from":"inventory","localField":"_id","foreignField":"linkedGroup","as":"inventory_links"}},{$addFields:{_id:1,inventory_links:{"$arrayElemAt":["$inventory_links",0]}},},{$project:{output:{$reduce:{input:"$inventory_links.list",initialValue:0,in:{$cond:[{$gte:["$$this.data.number","$$value"]},"$$this.data.number","$$value"]}}}}}])