RoR: Save information from 3 models at same time - ruby-on-rails

Question

I am trying to make it so that when I save an answer, I also save the prop_id that is associated with that answer.
I have a nested route relationship so that each prop (stands for proposition or bet) has a an associated answer like this: http://localhost:3000/props/1/answers/new.
Right now, when I save an answer, I save the answer choice and the user_id who created the answer. I need to save also the prop that is associated with the answer.
Answers Controller:
class AnswersController < ApplicationController
attr_accessor :user, :answer
def index
end
def new
#prop = Prop.find(params[:prop_id])
#user = User.find(session[:user_id])
#answer = Answer.new
end
def create
#prop = Prop.find(params[:prop_id])
#user = User.find(session[:user_id])
#answer = #user.answers.create(answer_params)
if #answer.save
redirect_to root_path
else
render 'new'
end
end
def show
#answer = Answer.find params[:id]
end
end
private
def answer_params
params.require(:answer).permit(:choice, :id, :prop_id)
end
Answer Model
class Answer < ActiveRecord::Base
belongs_to :prop
belongs_to :created_by, :class_name => "User", :foreign_key => "created_by"
has_many :users
end
Prop Model
class Prop < ActiveRecord::Base
belongs_to :user
has_many :comments
has_many :answers
end
User Model
class User < ActiveRecord::Base
has_many :props
has_many :answers
has_many :created_answers, :class_name => "Answer", :foreign_key => "created_by"
before_save { self.email = email.downcase }
validates :username, presence: true, uniqueness: {case_sensitive: false}, length: {minimum: 3, maximum: 25}
has_secure_password
end

Answer 1

Just modify your code a little bit, and it will work:
def create
#user = User.find(session[:user_id])
#prop = #user.props.find_by(id: params[:prop_id])
#answer = #user.answers.build(answer_params)
#answer.prop = #prop
# Modify #user, #prop or #answer here
# This will save #user, #prop & #answer
if #user.save
redirect_to root_path
else
render 'new'
end
end