Mongoose model get undefined properties after population

I got a problem for a basic request. All properties of a mongoose model I fetch are undefined in the exec() callback.

Here is my schema :

userSchema: new Schema({
    email: { type: String, limit: 50, index: true },
    password: String,
    birthdate: { type: Date },
    active: { type: Boolean, default: true },
    friends: [{
      _friend: { type: Schema.ObjectId, ref: 'User' },
      addedDate: { type: Date, default: Date.now }
    }],
    registrationDate: { type: Date, default: Date.now }
  })

You can already notice that my "friends" property is an array of objects referencing another schema.

Now here is my query :

dbModels.User
.find({ _id: req.session.user._id })
.populate('friends._friend', 'email birthdate')
.exec(function (err, _user){
  if (err || !_user){
    apiUtils.errorResponse(res, sw, 'Error when fetching friends.', 500);
  } else {

    console.log('user', _user);
    // This output the object with all its properties

    console.log('user birthdate', _user.birthdate);
    // _user.birthdate is undefined

    console.log('user friends', _user.friends);
    // _user.friends is undefined

    apiUtils.jsonResponse(res, sw, _user);
  }
});

When this web service return '_user', each properties are well defined and have the correct values. The problem is that I only want to return _user.friends which is not possible since it's undefined.

Now, here is apiUtils.jsonResponse function :

exports.jsonResponse = function (res, sw, body) {

  console.log(body.friends);
  // At this breakpoint, body.friends is still undefined

  (sw || _sw).setHeaders(res);
  if (util.isArray(body)) {
    for (var i = 0; i < body.length; i++) {
      body[i] = exports.cleanResults(body[i]);
    }
  } else {

    console.log(body.friends);
    // At this breakpoint body.friends is still undefined

    body = exports.cleanResults(body);
  }
  res.send(httpCode || 200, JSON.stringify(body));
};

And the cleanResults function :

exports.cleanResults = function (body) {

  console.log(body.friends);
  // At this point, body.friends is FINALLY DEFINED

  if (typeof body.toObject === 'function') {
    body = body.toObject();
    delete body.__v;
  }

  for (var attr in body) {
    if (body.hasOwnProperty(attr) && attr[0] == '_') {
      var _attr = attr.replace('_', '');
      body[_attr] = body[attr];
      delete body[attr];
    }
  }
  return body;
};

I tried to set a timeout to see if the problem came from async but it changed nothing. I'm a bit desesperate at this time and I wanted to know if you already encountered the same problem before ?