codeblock.rb

class Codeblock < ActiveRecord::Base
has_many :sections, as: :item
end

codeblocks_controller.rb

class CodeblocksController < ApplicationController

	before_action :correct_step, only: [:create, :destroy]

	def create
		@codeblock = @step.codeblocks.create(codeblock_params)
		if @codeblock.save
			@section = @step.sections.build
      		@section.item = @codeblock
      		@section.save
			flash[:success] = "CodeBlock added"
			
		else
			flash[:error] = "Failed to add CodeBlock"
		end
		redirect_to tutorial_step_url(@tutorial, @step)
	end

	# DELETE /tutorials/1
  # DELETE /tutorials/1.json
  def destroy
  	@codeblock = @step.codeblocks.find_by(id: params[:id])
    @codeblock.destroy
    respond_to do |format|
      format.html { redirect_to tutorial_step_url(@tutorial, @step), notice: 'CodeBlock was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

	private
		def correct_step
			@tutorial = current_user.tutorials.find_by(id: params[:tutorial_id])
			@step = @tutorial.steps.find_by(id: params[:step_id])
			redirect_to root_url if @step.nil? || @tutorial.nil?
		end

		def codeblock_params
			params.require(:codeblock).permit(:code, :description)
		end
end

gistfile1.txt

[45] pry(main)> @section = @step.sections.build
=> #<Section id: nil, step_id: 6, item_id: nil, item_type: nil, created_at: nil, updated_at: nil>
[46] pry(main)> @codeblock = Codeblock.build
NoMethodError: undefined method `build' for #<Class:0x00000005b4fac8>
from /home/vagrant/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.2/lib/active_record/dynamic_matchers.rb:26:in `method_missing'
[47] pry(main)> @codeblock = Codeblock.new
=> #<Codeblock id: nil, code: nil, description: nil, created_at: nil, updated_at: nil, step_id: nil>
[48] pry(main)> @codeblock.code = 'def end'
=> "def end"
[49] pry(main)> @codeblock.description = 'some text'
=> "some text"
[50] pry(main)> @codeblock.save
   (2.9ms)  begin transaction
  SQL (5.6ms)  INSERT INTO "codeblocks" ("code", "created_at", "description", "updated_at") VALUES (?, ?, ?, ?)  [["code", "def en
d"], ["created_at", "2014-12-11 07:37:09.296127"], ["description", "some text"], ["updated_at", "2014-12-11 07:37:09.296127"]]
   (5.4ms)  commit transaction
=> true
[51] pry(main)> @section.item = @codeblock
=> #<Codeblock id: 15, code: "def end", description: "some text", created_at: "2014-12-11 07:37:09", updated_at: "2014-12-11 07:37
:09", step_id: nil>
[52] pry(main)> @section.save
   (0.8ms)  begin transaction
  SQL (4.4ms)  INSERT INTO "sections" ("created_at", "item_id", "item_type", "step_id", "updated_at") VALUES (?, ?, ?, ?, ?)  [["c
reated_at", "2014-12-11 07:37:34.386648"], ["item_id", 15], ["item_type", "Codeblock"], ["step_id", 6], ["updated_at", "2014-12-11
 07:37:34.386648"]]
   (5.8ms)  commit transaction
=> true
[53] pry(main)> @section.item
=> #<Codeblock id: 15, code: "def end", description: "some text", created_at: "2014-12-11 07:37:09", updated_at: "2014-12-11 07:37
:09", step_id: nil>
[54] pry(main)>

section.rb

class Section < ActiveRecord::Base
  belongs_to :step
  belongs_to :item, polymorphic: true
end

What i thought i should expect

I have a section that has a polymorphic association with codeblock
@section = Section.first
it was created by
@codeblock = Codeblock.create(codeblock_params)
	if @codeblock.save
      		@section = @step.sections.build
      		@section.item = @codeblock
      		@section.save
    	end
Here is the section.
@section => #<Section id: 1, step_id: 6, item_id: 7, item_type: "Codeblock", created_at: "2014-12-11 05:09:26", updated_at: "2014-12-11 05:09:26">
I thought that if i then called 
@section.item I would get a codeblock from rails doing something like
Codeblock Load (4.1ms)  SELECT  "codeblocks".* FROM "codeblocks"  WHERE "codeblocks"."id" = ? LIMIT 1  [["id", 7]]
Instead I get 
[33] pry(main)> @section.item
=> nil