db={"categories":[{_id:1,name:"A",values:[{_id:"5f55949054f3f31db0491b5c",value:"PC"}]},{_id:2,name:"A",values:[{_id:"5f55949054f3f31db0491b5b",value:"LAPTOP"},{_id:"5f55949054f3f31db0491b5a",value:"LAPTOPA"}]}],"products":[{name:"My laptop",description:"Very ugly laptop",categories:["5f55949054f3f31db0491b5c","5f55949054f3f31db0491b5b"]}]}

db.products.aggregate([{$lookup:{from:"categories",let:{cat:"$categories"},as:"categories",pipeline:[{$unwind:"$values"},{$match:{$expr:{$in:["$values._id","$$cat"]}}},{$project:{_id:0,value:"$values.value"}}]}},])