add Taggings record to Post and Tag with Rails 4 nested form

I have 3 models, Post, Tag and Taggings and I am trying to build a form to create the taggings through an array of checkboxes. I can't seem to get ANY of the Tagging records to save (even though it

<%= form_for @post %>
<%= hidden_field_tag "post[tag_ids][]", nil %>
<% Tag.colors.each do |tag| %>
<%= check_box_tag  "post[tag_ids][]", tag.id,   @post.tag.include?(tag), id: dom_id(tag)
%> 
<%= label_tag dom_id(tag), tag.name %><br>
<% end %>
<label>Post title</label>
<%=    text_field :title %>

<%= submit_tag 'ADD TAGS' %> 

Controller

class PostsController < ApplicationController

def add_taggings 
    @post = Post.find(params[:id])
    @taggings = Tagging.new
    if @post.update_attributes(post_params)
        redirect_to  @Post
        flash[:notice] = "Successfully Tagged

    else
        render :edit
    end
end

  def post_params
        params.require(:post).permit( :id, :title, :img, :content, :author,
        taggings_attributes:[:id, :post_id, :tag_id, {:tags => []}, {:posts => []}, :_destroy])

Models

class Post < ActiveRecord::Base
    has_many :taggings

    class Tag < ActiveRecord::Base
    has_many :taggings

    class Tagging < ActiveRecord::Base
    belongs_to :tag
    belongs_to :post
    validates_uniqueness_of :tag, :scope => :post_id