Database
db={setting:[{"_id":ObjectId("5aff382669153dc20edf945a"),"key":"mgmt","site_id":"5aff382669153dc20edf9456","advanced_feature_enabled":false,"auto_upgrade":true}],site:[{"_id":ObjectId("5aff382669153dc20edf9456"),"desc":"Joe's Crab Shack",}]}
Query
db.site.aggregate([{$addFields:{siteIdString:{$toString:"$_id"}}},{$lookup:{from:"setting",localField:"siteIdString",foreignField:"site_id",as:"setting"}}])