mongoose 4.1+ maintainer suggestion:

es2015 (es6):

require('mongoose').Promise = Promise;

bluebird:

require('mongoose').Promise = require('bluebird');

Q:

require('mongoose').Promise = require('q').Promise;

I do not know moongose, but in general, functions like fail or catch are convenience shortcuts and are not a part of the promises spec. As such the library does not need to have them to be promises-compliant. Apparently in your case they are not there.

You can achieve same functionality with then(successHandler, rejectionHandler).

In order to handle the promise rejection, you can rewrite your code as follows:

User.findOne({ 'email' :  user_email }).exec() }).then (promisedTransformUserSchemaToFrontendObjectWithProjectMapping)   .then (function (feUser) {       return new Promise(function (resolve, reject) {          res.json(feUser);          return resolve(feUser);      });   }).then (undefined, function (err) {      console.log(err);      sendError(res,"failed to get user",err);   });

Another way to do it is shown in the bluebird docs:

https://github.com/petkaantonov/bluebird/blob/master/API.md#promiseresolvedynamic-value---promise

You can wrap the mongoose promise in bluebird's Promise.resolve(), and you will get back a bluebird promise.

 Promise.resolve(User.findOne({ 'email' :  user_email }).exec()) .then (promisedTransformUserSchemaToFrontendObjectWithProjectMapping)   .then (function (feUser) {          res.json(feUser);          return feUser;   }).fail/catch  (function (err) {      console.log(err);      sendError(res,"failed to get user",err);   });