Strong Params won't save nested has_many - ruby-on-rails-4

Question

I'm using Rails 4, and with it Strong Params. For some reason I can't save my nested parameters. I've looked at how to do it and even might've gotten it working in a different Rails app before. I know to permit draft_players_attributes and list all of it's accepted params in an array.
Here are the params coming in:
Started POST "/draft_groups" for 127.0.0.1 at 2013-12-04 22:55:32 -0500
User Load (1.6ms) SELECT "users".* FROM "users" WHERE "users"."id" = 1 ORDER BY "users"."id" ASC LIMIT 1
Processing by DraftGroupsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"2W3bs1U7+CEzsWl+jDi3xZi5CyldYeZXCz3KU6c+sYY=", "draft_group"=>{"name"=>"Best Group Ever", "draft_id"=>"3", "captain_id"=>"1"}, "draft_players"=>{"player_id"=>"1", "position"=>"Handler", "rating"=>"10", "info"=>"Smart"}, "commit"=>"Update"}
(0.3ms) BEGIN
SQL (2.6ms) INSERT INTO "draft_groups" ("captain_id", "created_at", "draft_id", "name", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id" [["captain_id", 1], ["created_at", Thu, 05 Dec 2013 03:55:32 UTC +00:00], ["draft_id", 3], ["name", "Best Group Ever"], ["updated_at", Thu, 05 Dec 2013 03:55:32 UTC +00:00]]
(0.5ms) COMMIT
Redirected to http://lvh.me:3000/drafts/3
Completed 302 Found in 8ms (ActiveRecord: 3.3ms)
Here's my controller:
class DraftGroupsController < ApplicationController
def create
#draft_group = DraftGroup.create(draft_group_params)
redirect_to :back, :notice => "Draft Group successfully created."
end
def update
#draft_group = DraftGroup.find(params[:id])
#draft_group.update(draft_group_params)
redirect_to :back, :notice => "Draft Group successfully updated."
end
def destroy
#draft_group = DraftGroup.find(params[:id]).destroy
redirect_to :back, :notice => "Draft Group successfully destroyed."
end
private
def draft_group_params
params.require(:draft_group).permit(:name,
:draft_id,
:captain_id,
draft_players_attributes:
[
:_destroy,
:id,
:player_id,
:position,
:rating,
:info
]
)
end
end
And my models:
class DraftGroup < ActiveRecord::Base
has_many :draft_players, :dependent => :destroy
belongs_to :captain, :class_name => "User"
accepts_nested_attributes_for :draft_players
end
class DraftPlayer < ActiveRecord::Base
belongs_to :draft_group
belongs_to :player, class_name: "User"
end
And my view:
<% #groups.each do |group| %>
<div class="span6 group">
<h4><%= "#{group.name}" %></h4>
<%= simple_form_for(group, :html => { :class => "auto-width" } ) do |f| %>
<div class="row">
<%= f.input :name, :label => false %>
<%= f.hidden_field :draft_id %>
<%= f.hidden_field :captain_id %>
</div>
<table>
<tr>
<th>Player</th>
<th>Position</th>
<th>Rating</th>
<th>Info</th>
</tr>
<%= simple_fields_for :draft_players do |player| %>
<tr>
<td><%= player.input :player_id, :label => false, :as => :select, :collection => User.active %></td>
<td><%= player.input :position, :label => false %></td>
<td><%= player.input :rating, :label => false %></td>
<td><%= player.input :info, :label => false %></td>
</tr>
<% end %>
</table>
<div class="row">
<%= f.button :submit, "Update", :class => "btn btn-primary" %>
</div>
<% end %>
</div>
<% end %>
EDIT: Added view code, and will probably take table out, shifting it into using bootstraps columns layout.

Answer 1

To make sure parameters are nested correctly (and so Rails can understand nested model attributes) you should call f.simple_fields_for(:draft_players) rather than simple_fields_for(:draft_players).
In other words, call simple_fields_for on the FormBuilder object f rather than calling the helper directly.
That way Rails can look up and validate the nested association correctly.