db={credentials:[{_id:"c1",title:"title"},{_id:"c2",title:"title 2"}],user:[{_id:"u1",credential_id:"c1",created_at:"",identifier:{first_name:"f name",middle_name:"m name",last_name:"l name",},},{_id:"u2",credential_id:"c2",created_at:"",identifier:{first_name:"f name",middle_name:"m name",last_name:"l name",},},]}

db.user.aggregate([{$lookup:{from:"credentials",localField:"credential_id",foreignField:"_id",as:"user_credential"}},{$unwind:"$user_credential"},{$project:{_id:0,user_credential_id:"$credential_id",member:{first_name:"$identifier.first_name",middle_name:"$identifier.middle_name",last_name:"$identifier.last_name",title:"$user_credential.title",created_at:"$created_at",}}}])