db={setting:[{"_id":ObjectId("5aff382669153dc20edf945a"),"key":"mgmt","site_id":ObjectId("5aff382669153dc20edf9456"),"advanced_feature_enabled":false,"auto_upgrade":true}],site:[{"_id":ObjectId("5aff382669153dc20edf9456"),"desc":"Joe's Crab Shack",}]}

db.site.aggregate([{$lookup:{from:"setting",localField:"_id",foreignField:"site_id",as:"setting"}}])