node.js - Why can't I update this MongodDB document in the first callback function (while I'm able to update a different document without issues)? -

sorry if title confusing, had hard time summarizing problem in single question. implementing comment system generic article posting/reading web app using mean stack.

i have models author, article, , comment following (omitting irrelevant fields createdby):

var authorschema = new schema({     // removing unnecessary fields       articles: [{         type: schema.objectid,         ref: 'article'     }],     comments: [{     // want find of users comments         type: schema.objectid,             ref: 'comment'     }]     ... }  var commentschema = new schema({     // removing unnecessary fields       content: {         type: string     },     author: {  // want able find comment's author         type: schema.objectid,         ref: 'author'     },     ... }  var articleschema = new schema({     content: {         type: string     },     author: {         type: schema.objectid,         ref: 'author'     },     comments: [{         type: schema.objectid,         ref: 'comment'     }]     ...        } 

here code creating comment in comment controller:

commentctrl.createcomment = function(req, res, next) { var comment = new comment(req.body); if (req.user) {     comment.author = req.user;     // desired article stored on req object earlier middleware     req.article.comments.unshift(comment); // naive way add comment     req.article.save(         function(err) {         if (err) {             return senderror(res, 400, geterrormessage(err));         } else {             author.findbyidandupdate(                 req.user._id,                 {$push: {comments: comment}},                 function(err){                     res.json(comment);                 }             )         }     });  } else {     return senderror(res, 401,'the user not logged in.'); } }; 

so weird part. author correctly storing comment in comments array field whereas article not. after execution, here console log of article references author (again, removed irrelevant fields clarity):

{ _id: 56a7b5028bffef9e1ba58ab4,   author:     { _id: 56a7b1791a5f728c1bb1ee99,      username: 'xtest1',      comments:        [ 53a78291a5f728c1bbcef9b ],      articles:        [ 53a78291a5f728c1bbcef9a ],    },   title: 'test article',   content: '...',   comments: [] } 

my original thought naive approach of unshifting comment comments array not working, while had article object on req object, changed implementation mimic code seems working updating author:

article.findbyidandupdate(     req.article._id,     {$push: {comments: comment}},     function(err){ ... // same inner function above (to update author)     } } 

that didn't change @ all. i'm not getting error messages (which makes sense since inner callback working fine), have no idea what's going on.

i figured out problem, , feel bit silly. answering own question, can prevent others same mistake.

i not creating comment document in database. creating comment in application, storing references (or rather, trying to) in author , article documents.

the solution first save comment in db, carry out original code in successful callback function.

the 1 thing can't figure out why storing reference in author object @ all? if has ideas might shed light on dormant problem can't foresee.