Database
db={"STAT_GROUP":[{"_id":38282922829,"statValues":[{_ref:282828290,}]}],"STAT_DATA":[{"_id":282828290,"key":"123"}]}
Query
db.STAT_GROUP.aggregate([{$unwind:"$statValues"},{$lookup:{from:"STAT_DATA",localField:"statValues._ref",foreignField:"_id",as:"Refs"}}])