Database
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":3}}]}]}
Query
db.groups.aggregate([{"$lookup":{"from":"inventory","localField":"_id","foreignField":"linkedGroup","as":"inventory_links"}},{$group:{_id:1,output:{$max:"$inventory_links.list.data.number"},},}])