Nested Form example for http://www.reddit.com/r/rails/comments/29fgf0/50_stackoverflow_anyone_got_an_answer/

new.html.haml

%h1 New Team

= form_for @team do |f|

  = f.label :name
  = f.text_field :name

  -# fields_for accepts a second param of the object. Use @team.players incase its an edit, otherwise build a new player
  = f.fields_for :players, (@team.players || @team.players.build) do |players|
    = players.label :player_name
    = players.text_field :name

  = f.submit "Create Team"

teams_controller.rb

class TeamsController < ApplicationController
  def new
    @team = Team.new
  end

  def create
    @team = current_user.build_team(team_params)

    if @team.save
      redirect_to teams_path
    else
      render :new
    end
  end

  private

  def team_params
    params.require(:team).permit(:name, players_attributes: [:id, :name, :role])
  end
end