[RUBY] [Rails trivia] Even if you write blog_path instead of blog_path (@ blog), id is automatically completed.

For example, suppose you have the following blogs/edit.html.erb as a Rails View file:

<h1>Editing Blog</h1>

<%= render 'form', blog: @blog %>

<%= link_to 'Show', blog_path(@blog) %> |
<%= link_to 'Back', blogs_path %>

The 5th line blog_path (@blog) is a helper method that generates a path like / blogs/10. (Here, it is assumed that the value of @ blog.id is 10.)

As you can see, you would normally pass an instance of ActiveRecord like @ blog as an argument to blog_path. However, calling blog_path without arguments as follows does not cause an error.

<%= link_to 'Show', blog_path %> |

The generated path will be / blogs/10 as before.

The same is true not only in View but also in the controller.

def update
  if @blog.update(blog_params)
    # blog_path(@blog)Without writing/blogs/Redirected to 10
    redirect_to blog_path, notice: '...'
  else
    render :edit
  end
end

Why?

I think that the above edit screen is usually called with the following path (URL).

/blogs/10/edit

If you call a helper method with no arguments like blog_path,/blogs/10 will be generated by inheriting the id (10 in this case) included in the path at the time of request.

Case where an error occurs if there is no argument

If you write app.blog_path in rails console etc., an error will occur because there is no id to inherit.

> app.blog_path
Traceback (most recent call last):
        1: from (irb):2
ActionController::UrlGenerationError (No route matches {:action=>"show", :controller=>"blogs"}, missing required keys: [:id])
Did you mean?  blog_url
               blogs_url
               blogs_path
               new_blog_url

Is it ant or pear to use blog_path with no arguments?

Personally, I don't think so.

This is because it is safer to write blog_path (@blog) to explicitly indicate that "the path you want to generate with this method is this id (this object)" rather than implicitly determining the id. ..

Also, if there is a habit of omitting arguments, a path may be generated with an unintended id depending on the logic of View, which may cause unexpected problems.

Where is the documentation?

There was such a description in the documentation of the url_for method.

https://edgeapi.rubyonrails.org/classes/ActionDispatch/Routing/UrlFor.html#method-i-url_for

Missing routes keys may be filled in from the current request's parameters (e.g. :controller, :action, :id and any other parameters that are placed in the path).

(Author's translation) The missing routing key is obtained from the request parameters (eg : controller,: action, : id, and other parameters contained in the path).

As stated in the above documentation, writing url_for (controller:'blogs', action:'show') instead of blog_path will generate/blogs/10 as well. I will. (More specifically, just url_for (action:'show') is OK)

Confirmed version

• Rails 6.0.3.3