blog.ex

def create_comment(attrs \\ %{}) do
  %Comment{}
  |> Comment.changeset(attrs)
  # TODO how to I get "post" here, to do the following step?
  # |> Ecto.Changeset.put_assoc(:post, post)
  |> Repo.insert()
end

comment_controller.ex

def create(conn, %{"comment" => comment_params, "post_id" => post_id}) do
    changeset = %Comment{}
      |> Comment.changeset(comment_params)
      |> Ecto.Changeset.put_change(:post_id, String.to_integer(post_id)) # TODO this works, but ugly string to integer?

    case Staticblog.Repo.insert(changeset) do
      {:ok, comment} ->
        conn
        |> put_flash(:info, "Comment created successfully.")
        |> redirect(to: Routes.post_path(conn, :show, post_id))

      {:error, %Ecto.Changeset{} = changeset} ->
        render(conn, "new.html", changeset: changeset)
    end
  end

post_controller.ex

def show(conn, %{"id" => id}) do
  post = Blog.get_post!(id)
    |> Repo.preload(:comments)
  changeset = Blog.change_comment(%Comment{}) # TODO maybe squeeze in the post.id here?
  render(conn, "show.html", post: post, changeset: changeset)
end

show.html.eex

<h1>Show Post</h1>

<ul>

  <li>
    <strong>Title:</strong>
    <%= @post.title %>
  </li>

  <li>
    <strong>Body:</strong>
    <%= @post.body %>
  </li>

</ul>

<strong>Comments:</strong>
<table>
  <%= for comment <- @post.comments do %>
      <tr>
        <td><%= comment.text %></td>

        <td>
          <span><%= link "Show", to: Routes.post_comment_path(@conn, :show, @post, comment) %></span>
          <span><%= link "Edit", to: Routes.post_comment_path(@conn, :edit, @post, comment) %></span>
          <span><%= link "Delete", to: Routes.post_comment_path(@conn, :delete, @post, comment), method: :delete, data: [confirm: "Are you sure?"] %></span>
        </td>
      </tr>
  <% end %>
</table>

<%= render StaticblogWeb.CommentView, "form.html", Map.put(assigns, :action, Routes.post_comment_path(@conn, :create, @post)) %>

<span><%= link "Edit", to: Routes.post_path(@conn, :edit, @post) %></span>
<span><%= link "Back", to: Routes.post_path(@conn, :index) %></span>