db={"schools":[{"name":"school1","sections":[{_id:"100","name":"section1","cabinets":[{"name":"cabinet1","columns":[{"_id":"301","index":1,}]}]}]},],"lockers":[{"_id":"200","columnId":"301"},{"_id":"201","columnId":"301"},{"_id":"202","columnId":"301"}]}

db.schools.aggregate([{$unwind:{path:"$sections",},},{$unwind:{path:"$sections.cabinets",},},{$unwind:{path:"$sections.cabinets.columns",},},{$lookup:{from:"lockers",localField:"sections.cabinets.columns._id",foreignField:"columnId",as:"sections.cabinets.columns.lockers",},}])