How to fix the Converting circular structure to JSON error when displaying all elements of a collection from MongoDB, as well as individual fields through mongoose?

Question

JavaScript

I want to return it is simple to return certain objects from the collection, but this does not work. Did everything as in the example.

Error code:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'ReplSet'
    |     property 's' -> object with constructor 'Object'
    |     property 'coreTopology' -> object with constructor 'ReplSet'
    |     ...
    |     property 's' -> object with constructor 'Object'
    --- property 'topology' closes the circle

My GET method:

router.get('/assignments', auth.required, function (req, res, next) {
  var user = User.findById(req.payload.id);

  if (!user) { return res.sendStatus(401); }

  var assignments = Assignment.find({ author: user });

  return res.json({
    assignments: assignments.map(function (assignment) {
      return assignment.toJSONFor(user);
    })
  })
})

JSON generation method in Mongoose model

AssignmentSchema.methods.toJSONFor = function(user){
  return {
    slug: this.slug,
    publicSlug: this.publicSlug,
    title: this.title,
    description: this.description,
    createdAt: this.createdAt,
    updatedAt: this.updatedAt,
    attempts: this.attempts,
    questions: this.questions,
    author: this.author.toProfileJSONFor(user)
  }
};

Do you have any ideas how to simplify this or get rid of the error?