DRY controller...

items_controller.rb

class ItemsController < ApplicationController

  def index
    @items = Item.all
  end

  def create
    @item = Item.new(allowable_params)
    update_or_create @item, :save
  end

  def update
    @item = Item.find(params[:id])
    update_or_create @item, :update_attributes
  end

  private

  def update_or_create(item, action)
    if item.send action, allowable_params
      render "items/show"
    else
      render json: item.errors.messages, status: :bad_request
    end
  end

  def allowable_params
    params.require(:item).
      permit \
        :body,     :priority,
        :title,    :complete,
        :position
  end
end