Add category_name to products index

index.html.eex

<h2>Listing products</h2>

<table class="table">
  <thead>
    <tr>
      <th>Name</th>
      <th>Price</th>
      <th>Category</th>

      <th></th>
    </tr>
  </thead>
  <tbody>
<%= for product <- @products do %>
    <tr>
      <td><%= product.name %></td>
      <td><%= product.price %></td>
      <td><%= product.category.category_name %></td> <!-- Update this line -->

      <td class="text-right">
        <%= link "Show", to: product_path(@conn, :show, product), class: "btn btn-default btn-xs" %>
        <%= link "Edit", to: product_path(@conn, :edit, product), class: "btn btn-default btn-xs" %>
        <%= link "Delete", to: product_path(@conn, :delete, product), method: :delete, data: [confirm: "Are you sure?"], class: "btn btn-danger btn-xs" %>
      </td>
    </tr>
<% end %>
  </tbody>
</table>

<%= link "New product", to: product_path(@conn, :new) %>

rummage_example_products_controller.ex

defmodule RummageExample.ProductController do
  use RummageExample.Web, :controller

  alias RummageExample.Product

  def index(conn, _params) do
    products = Product
      |> Repo.all
      |> Repo.preload(:category) # <-- Add this

    render(conn, "index.html", products: products)
  end

  def new(conn, _params) do
    changeset = Product.changeset(%Product{})
    render(conn, "new.html", changeset: changeset)
  end

  def create(conn, %{"product" => product_params}) do
    changeset = Product.changeset(%Product{}, product_params)

    case Repo.insert(changeset) do
      {:ok, _product} ->
        conn
        |> put_flash(:info, "Product created successfully.")
        |> redirect(to: product_path(conn, :index))
      {:error, changeset} ->
        render(conn, "new.html", changeset: changeset)
    end
  end

  def show(conn, %{"id" => id}) do
    product = Repo.get!(Product, id)
    render(conn, "show.html", product: product)
  end

  def edit(conn, %{"id" => id}) do
    product = Repo.get!(Product, id)
    changeset = Product.changeset(product)
    render(conn, "edit.html", product: product, changeset: changeset)
  end

  def update(conn, %{"id" => id, "product" => product_params}) do
    product = Repo.get!(Product, id)
    changeset = Product.changeset(product, product_params)

    case Repo.update(changeset) do
      {:ok, product} ->
        conn
        |> put_flash(:info, "Product updated successfully.")
        |> redirect(to: product_path(conn, :show, product))
      {:error, changeset} ->
        render(conn, "edit.html", product: product, changeset: changeset)
    end
  end

  def delete(conn, %{"id" => id}) do
    product = Repo.get!(Product, id)

    # Here we use delete! (with a bang) because we expect
    # it to always work (and if it does not, it will raise).
    Repo.delete!(product)

    conn
    |> put_flash(:info, "Product deleted successfully.")
    |> redirect(to: product_path(conn, :index))
  end
end