Rails4でcollection_check_boxesを使って、多対多の関連をチェックボックスで設定する ref: http://qiita.com/hamichamp/items/8cf980e6d5ca9fd7e96f

_form.html.erb

  ...

  <div class="field">
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :authors %><br />
    <%= collection_check_boxes(:post, :author_ids, Author.all, :id, :name) do |b| %>
      <%= b.label { b.check_box + b.text } %>
    <% end %>
  </div>

  ...

author.rb

class Author < ActiveRecord::Base
  has_many :author_posts
  has_many :posts, :through => :author_posts
end

file0.txt

rails g scaffold post title:string
rails g scaffold author name:string
rails g scaffold author_post post:references author:references

file4.txt

rake db:migrate
rails s

post.rb

class Post < ActiveRecord::Base
  has_many :author_posts
  has_many :authors, :through => :author_posts
end

posts_controller.rb

class PostsController < ApplicationController

    ...

    def post_params
      params.require(:post).permit(:title, {:author_ids => []})
    end

    ...

end