Create json nested attributes

controllers.rb

class UsersController < ApplicationController
  def create
    @user = User.new(user_params)

    respond_to do |format|
      if @user.save
        format.html { redirect_to @user, notice: 'User was successfully created.' }
        format.json { render :show, status: :created, location: @user }
      else
        format.html { render :new }
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
  end
  
  private
  def user_params
      params.require(:user).permit(
      :first_name, 
      :last_name, 
      phones_attributes: [:user_id,:tel]
    end
end

models.rb

class Phone < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :phones
  accepts_nested_attributes_for :phones
end

return.json

//I am not passing anything to phones[]
{
    "id": 42,
    "first_name": null,
    "last_name": null,
    "phone": "97226491304",
    "balance": null,
    "password_digest": "1234",
    "created_at": "2015-08-19 19:54:20 UTC",
    "phones": [],
    "courses": [],
    "course_count": 0
}

shell

curl -H "Content-Type: application/json" -X POST -d '{"first_name":"yoyo","last_name":"popo","phones_attributes":[{"tel":"12127773456"}]}' http://localhost/users.json