class User < ActiveRecord::Base
  has_many :participations, foreign_key: :participant_id
  has_many :owned_tasks, class_name: "Task", foreign_key: :owner_id
end

class Participation < ActiveRecord::Base
  enum status: [:interested, :selected]

  belongs_to :task
  belongs_to :participant, class_name: "User"
end

class Task < ActiveRecord::Base
  enum status: [:open, :in_progress, :complete]

  has_many :participations
  has_many :participants, through: :participations, source: :participant
  # Dynamically generates relations such as 'selected_participants'
  Participation.statuses.keys.each do |status|
    has_many "#{status}_participants".to_sym,
    -> { where(participations: { status: status.to_sym }) },
    through: :participations,
    source: :participant
  end

  belongs_to :owner, class_name: "User"
end

user = User.first
task = Task.first
user.owned_tasks << task

user_2 = User.find(2)
task.participants << user_2

<% unless current_user == @task.owner %>
  <div class="volunteer-form">
    <% if current_user.is_participating? %>
        <%= render 'cancel' %>
    <% else %>
        <%= render 'volunteer' %>
    <% end %>
 </div>
<% end %>

<%= form_for(current_user.participations.build(participant_id: current_user), remote: true) do |f| %>
    <div><%= f.hidden_field :participant_id %></div>
    <%= f.submit "Volunteer" %>
 <% end %>

<%= form_for(current_user.participations.find_by(participant_id: current_user), html: { method: :delete }, remote: true) do |f| %>
   <%= f.submit "Cancel" %>
<% end %>

// create.js.erb
$(".volunteer-form").html("<%= escape_javascript(render('tasks/volunteer')) %>");

// destroy.js.erb
$(".volunteer-form").html("<%= escape_javascript(render('tasks/cancel')) %>");