db={"Product":[{_id:1,name:"Great Product",regions:[11,12]}],"Region":[{_id:11,name:"Europe"},{_id:12,name:"North America"},{_id:13,name:"Middle East"}]}

db.Region.aggregate([{"$lookup":{"from":"Product","let":{id:"$_id"},"pipeline":[{$match:{$expr:{$in:["$$id","$regions"]}}},{$project:{_id:1,name:1}}],"as":"product"}},{"$unwind":{"path":"$product","preserveNullAndEmptyArrays":true}}])