db={"Category":[{code:"drink",name:"Soft Drink and Beer"},{code:"fast-food",name:"Burger and Chicken Fry"}],"GroupPeople":[{categories:{"drink":5,"fast-food":3}},{categories:{"drink":2}}]}

db.GroupPeople.aggregate([{$addFields:{categories:{$objectToArray:"$categories"}}},{$unwind:"$categories"},{$lookup:{from:"Category",let:{category:"$categories.k"},pipeline:[{$match:{$expr:{$eq:["$$category","$code"]}}}],as:"category"}},{$unwind:"$category"},{$group:{_id:"$_id",categories:{$push:{k:"$category.name",v:"$categories.v"}}}},{$project:{categories:{$arrayToObject:"$categories"}}}])