db={locations:[{_id:1,"country":"france"},{_id:2,"country":"france"},{_id:3,"country":"france"},{id:4,"country":"usa"}],products:[{_id:1,"locId":1},{_id:2,"locId":2},{_id:3,"locId":3},{_id:4,"locId":4}]}

db.locations.aggregate([{$match:{country:"france"}},{$lookup:{from:"products",as:"products",localField:"_id",foreignField:"locId"}},{$unwind:"$products"},{$group:{_id:null,products:{$push:"$products"}}},{$unwind:"$products"},{$project:{_id:"$products._id",locId:"$products.locId"}}])