block12.rb

<% if flash.present? %>
  <% flash[:errors].each do |msg| %>
    <%= content_tag :div, msg %>
  <% end %>
<% end %>

<%= form_with model: @post, local: true do |f| %>
  <%= f.label :title %>
  <%= f.text_field :title %>

  <%= f.label :body %>
  <%= f.text_area :body %>

  <%= f.select :category_id do %>
    <%= options_from_collection_for_select(Category.all, :id, :name) %>
  <% end %>

  <%= f.submit %>
<% end %>