Rails Devise methods & helpers don't work

Question

I have inherited an application built with Rails (v. 4.2.0) + AngularJS. Most of it works reasonably well, but I have hit a wall with Devise (v .3.5.1) that I am unable to overcome.

The application is structured this way: Angular.js handles the client side and is served from the back by the Rails app through an API. (The structure is very similar to the one from this tutorial: https://thinkster.io/angular-rails).

The problem if that for some reason, the app does not recognize helper methods of devise such as: current_user or user_signin? among others. Currently, anyone could make a request to /api/users and get a JSON with the information.

For instance, if we add this code to the /controllers/api/employees_controller.rb and you make a request in your browser to /api/employees/:id you get the JSON despite nos being logged

def show
  @employee = Employee.find(params[:id])
  respond_to do |format|
    format.json{
      if user_signed_in? then
        render text: @employee.to_json, status: 200
      else
        render :json => nil, status: 422
      end
    }
  end
end

Note: in this application the entity "Employee" is a type of "User"

I have tried solutions named in this post Rails devise: user_signed_in? not working but it didn't work for me

Here are all the parts of the code that I think could offer some light to this issue

/controllers/api/users/sessions_controller.rb

module Api
  class Users::SessionsController < Devise::SessionsController
  before_filter :configure_sign_in_params, only: [:create]

    #Devise couldn't find "users_url" so just defining it here (500 error)
    def users_url
      :root_path
    end

    # GET /resource/sign_in
    def new
      super
    end

    # POST /resource/sign_in
    def create
      user = User.new
      if ( params[:user][:email] == "" || params[:user][:password] == "") then
    render json: {errors: "Insufficient data provided (user and/or password)"}.to_json, status: :bad_request
      elsif User.find_by_email(params[:user][:email]).present?
    user = User.find_by_email(params[:user][:email])
    if user.valid_password?(params[:user][:password]) then
      sign_in :user, user
      render json: user.to_json(only: [:id, :email, :name, :last_name, :type, :company_id,:configuration,:phone]), status: :created
    else
      render json: {errors: "Wrong password."}.to_json, status: :unauthorized
    end
      else
    render json: {errors: "Could not find user with that email."}.to_json, status: :unauthorized
      end
    end

    # DELETE /resource/sign_out
    def destroy
      puts "HEEEEY"
    end

    # protected

    # If you have extra params to permit, append them to the sanitizer.
    def configure_sign_in_params
      devise_parameter_sanitizer.for(:sign_in) << :attribute
    end
  end
end

models/user.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable#, :confirmable

  scope :admins, -> { where(admin: true) }

  validates :email, presence: true
  validates :name, presence: true, length: { in: 1..50 }

end

config/routes.rb

Rails.application.routes.draw do

 namespace :api, defaults: {format: :json} do

   resources :contacts
   resources :job_application
  #  devise_for :admins, controllers: {sessions: 'admins/sessions'}
  #  devise_for :employees, controllers: {sessions: 'employees/sessions'}

    devise_for :users, controllers: {
    sessions: 'api/users/sessions',
    registrations: 'api/users/registrations',
    }

    devise_for :employees, controllers: {
       sessions: 'api/employees/sessions',
       registrations: 'api/employees/registrations',
     }

If you need any other part of the code, please just let me know.

Thank you very much!