form_for nested resources

# Comment Form
<%= form_for [@post, Comment.new] do |f| %>
  <%= f.text_field :body, placeholder: "Comment", class: "form-control", required: true, size: "30x10" %>
  <%= f.submit %>
<% end %>
  
# Post Controller
def show
  @post = Post.find(params[:id])
end
  
# Routes.rb
resources :posts do
  resources :comments
end