## category.rb (Model)
class Category
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  has n, :categorizations
  has n, :posts, :through => :categorizations

end

## categorization.rb (Model)
class Categorization
  include DataMapper::Resource

  property :id, Serial
  property :created_at, DateTime

  belongs_to :category
  belongs_to :post

end

## post.rb (Model)
class Post
  include DataMapper::Resource

  property :id, Serial
  property :title, String
  property :body, Text
  property :created_at, DateTime

  has n, :categorizations
  has n, :categories, :through => :categorizations

  has n, :comments

end

## posts.rb (select actions)
  def new
    @categories = Category.all
    only_provides :html
    @post = Post.new
    display @post
  end

  def create(post)
    cats = post['categories']
    post.delete('categories')
    @post = Post.new(post)
    if @post.save
      # This is where I am having my problems.
      # I need to be able to save the existing categories to the @post.
      # I have been able to establish that "Category.all(:id => cats)" returns as expected with the correct categories.
      @post.categories = Category.all(:id => cats)
      @post.save
      redirect resource(@post), :message => {:notice => "Post was successfully created"}
    else
      message[:error] = "Post failed to be created"
      render :new
    end
  end

## new.html.haml (New Post View)
%h1 New Post

= error_messages_for :post

= form_for(@post, :action => url(:posts) ) do
  %p= text_field :title, :label => "Title"
  %p= text_area :body, :label => "Body"
  %p= select :categories, :collection => @categories.map{|c| [c.id, c.name]}, :multiple => true

  %p= submit "Create"

= link_to 'Back', url(:posts)