Mongoid::Errors::MixedRelations in AnswersController#create - ruby-on-rails

Question

Getting following error msg while saving an answer:
Problem: Referencing a(n) Answer document from the User document via a relational association is not allowed since the Answer is embedded. Summary: In order to properly access a(n) Answer from User the reference would need to go through the root document of Answer. In a simple case this would require Mongoid to store an extra foreign key for the root, in more complex cases where Answer is multiple levels deep a key would need to be stored for each parent up the hierarchy. Resolution: Consider not embedding Answer, or do the key storage and access in a custom manner in the application code.
Above error is due to the code #answer.user = current_user in AnswersController.
I want to save the login username to the answer which is embaded in question.
deivse User model:
class User
include Mongoid::Document
has_many :questions
has_many :answers
class Question
include Mongoid::Document
include Mongoid::Timestamps
include Mongoid::Slug
field :title, type: String
slug :title
field :description, type: String
field :starred, type: Boolean
validates :title, :presence => true, :length => { :minimum => 20, :allow_blank => false }
embeds_many :comments
embeds_many :answers
#validates_presence_of :comments
belongs_to :user
end
class Answer
include Mongoid::Document
include Mongoid::Timestamps
field :content, type: String
validates :content, :presence => true, :allow_blank => false
embedded_in :question, :inverse_of => :answers
#validates_presence_of :comments
belongs_to :user
end
class AnswersController < ApplicationController
def create
#question = Question.find(params[:question_id])
#answer = #question.answers.create(params[:answer].permit(:answerer, :content))
#answer.user = current_user
redirect_to #question, :notice => "Answer added!"
end
end
Using Rails 4, Ruby 2.2.2, Mongoid.

Answer 1

That's exactly what the error message says.
Your Answer model is embedded in the question model. That is to say, you can only perform "normal" queries on the Question documents, and not on the models embedded in this one (actually you can, but it's more difficult and somehow kills the point of using embedded documents).
So you can get the user for a given answer, but not the inverse, which you have declared in your user model.
The simplest solution is to remove has_many :answers from the user model, but if you want to retrieve the list of answers for a given user, then embedding models is probably not the best solution: you should have relational models.
To make things clear, you should write belongs_to :user, inverse_of: nil