db={"collection":[{"weeks":["619e87d7b1bd6501c7aae286","619e87d7b1bd6501c7aae287"]}],"weeks":[{"_id":"619e87d7b1bd6501c7aae286","name":"week-1","description":"Commodo in o.","course":"619e87d7b1bd6501c7aae281","days":["619e87dab1bd6501c7aae2a8","619e87dab1bd6501c7aae2a9","619e87dab1bd6501c7aae2aa","619e87dab1bd6501c7aae2ab","619e87dab1bd6501c7aae2ac","619e87dab1bd6501c7aae2ad"],"isCopy":false,"__v":0},{"_id":"619e87d7b1bd6501c7aae287","name":"week-2","description":"Irure e.","course":"619e87d7b1bd6501c7aae281","days":["619e87dab1bd6501c7aae2db","619e87dab1bd6501c7aae2dc","619e87dab1bd6501c7aae2dd","619e87dab1bd6501c7aae2de","619e87dab1bd6501c7aae2df","619e87dab1bd6501c7aae2e0"],"isCopy":false,"__v":0}]}

db.collection.aggregate([{$lookup:{from:"weeks",let:{wks:"$weeks"},pipeline:[{$match:{$expr:{$in:["$_id",["$$wks"]]}}}],as:"weeks"}}])