db={"users":[{id:"a1b2c3",_p_store:"Stores$x1y2z3"},{id:"a1b2c3d5e6f7",_p_store:"Stores$x1y2z3a5b6d7"}],"stores":[{id:"x1y2z3",_p_user:"Users$a1b2c3"},{id:"x1y2z3a5b6d7",_p_user:"Users$a1b2c3d5e6f7"}]}

db.users.aggregate([{$match:{id:"a1b2c3"}},{$set:{store:{$substrBytes:["$_p_store",7,-1]}}},{$lookup:{from:"stores",localField:"store",foreignField:"id",as:"store"}},{$unwind:"$store"}])