Rails Nested Attributes Form

class Project < ActiveRecord::Base
# Relationship 
#belongs_to :role
has_many :roles
belongs_to :user

#has_many :users, :through => :roles

accepts_nested_attributes_for :roles
attr_accessible :project_name, :sprint_duration, :roles_attributes

class Role < ActiveRecord::Base
attr_accessible :project_id, :user_id, :user_role

belongs_to :project
end

<%= form_for Project.new do |f| %>
<%= f.label :project_name %>
<%= f.text_field :project_name %>
<%= f.fields_for :roles do |builder| %>
<%= builder.label :user_role %>
<%= builder.text_field :user_role %>
<% end %>
<%= f.submit %>
<% end %>