new.html.erb

<%= form_for(@production) do |form| %>
  <%= form.label :title, 'Title' %>
  <%= form.text_field :title %>

  <%= form.fields_for :employee_roles do |role_form| %>
    <%= form.label :employee_id %></dt>
    <%= form.select :employee_id, ... %>
    <%= form.label :role_id %></dt>
    <%= form.select :role_id, ... %>
  <% end %>
<% end %>

production.rb

class Production < ActiveRecord::Base

  has_many :employee_roles, :class_name => "ProductionEmployeeRole", :foreign_key => "production_id", :inverse_of => :production

  accepts_nested_attributes_for :employee_roles, :allow_destroy => true

end

production_employee_role.rb

class ProductionEmployeeRole < ActiveRecord::Base

  belongs_to :production, :inverse_of => :employee_roles
  belongs_to :employee, :inverse_of => :production_roles
  belongs_to :role, :class_name => "ProductionRole", :foreign_key => "role_id", :inverse_of => :production_employee_roles

  # The requirement of production_id results in the model not being valid when created together with a new production
  validates_presence_of :role_id, :production_id, :employee_id
  
end