class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, through: :memberships
  has_many :lists
  accepts_nested_attributes_for :lists
  cattr_accessor :current_user
end

class List < ApplicationRecord
  belongs_to :group
  validates_presence_of :group_id
end

<% @group.lists.each do |elem| %>
<p><strong><%= elem.title %></strong></p>
<p><%= elem.body %></p>
<%= link_to 'Delete list', [elem.group, elem], method: :delete,
            data: {confirm: "Are you sure?"} %>

  <%= link_to 'Update list', edit_group_list_path(:group_id => @group.id, :list_id => elem.id), method: :put %>

<% end %>

resources :groups do
resources :lists
end