undefined method `to_model' in rails

Question

I have the following ActiveAdmin model

class AdminUser < ActiveRecord::Base
has_many :challenges

  devise :database_authenticatable, 
         :recoverable, :rememberable, :trackable, :validatable
end

and the following Challenge model

class Challenge < ActiveRecord::Base
    belongs_to :subdomain
    belongs_to :admin_user

    has_many :comments, :dependent => :destroy

end

I defined the following method in my application.html.erb

 helper_method :all_admin_users

  def all_admin_users

    @users = AdminUser.all
  end

Adminuser has the following attributes :id,:name,:email Now when i try to access the challenges of a particular User

<ul>
<% all_admin_users.each do |user| %>
<li> <%= link_to user.name ,user.challenges%></li>
<br />
<% end %>
</ul>

I get the following error "undefined method `to_model' for Challenge::ActiveRecord_Associations_CollectionProxy

here's my routes.rb file

Rails.application.routes.draw do
  resources :comments
  devise_for :admin_users, ActiveAdmin::Devise.config

  get 'subdomains/index'

  get 'subdomains/edit'

  get 'subdomains/new'

  get 'subdomains/show'

  get 'home/index'




 root 'home#index'


 resources :challenges

 resources :subdomains
  ActiveAdmin.routes(self)
end

Answer 1

First in your routes file you need to add action to list users-challenged

In routes.rb at end add

get 'users/:user_id/challenges' => 'challenges#user_challenges', as: :user_challenges
# this way you will have action to view users challenges
# inside your challenges controller

In your challenges_controller.rb add action user_challenges

def user_challenges
  @challenges = Challenge.where(admin_user_id: params[:user_id])
end

Create view for this action, and display your challenges

Then you can create link for this action

<ul>
  <% all_admin_users.each do |user| %>
    <li> <%= link_to user.name , user_challenges_path(user.id)%> </li>
    <br />
  <% end %>
</ul>

Answer 2

Instead of user.challenges use user.challenge

UPDATE

<ul>
  <% all_admin_users.each do |user| %>
    <% user.challenges.each do |challenge| %>
      <li> <%= link_to user.name ,challenges_path(user_id: user.try(:id)) %></li>
      <br />
    <% end %>
  <% end %>
</ul>

challenges_controller.rb

def index
  @challenges = Challenge.where(admin_user_id: params[:user_id])
end