In Ruby on Rails, how to render JSON?

Sooner or later, most Rails developers will see the error message “Can only render or redirect once per action”. While this is annoying, it’s relatively easy to fix. Usually it happens because of a fundamental misunderstanding of the way that render works.

Evaluates to true, Rails will start the rendering process to dump the @book variable into the special_show view. But this will not stop the rest of the code in the show action from running, and when Rails hits the end of the action, it will start to render the regular_show view – and throw an error. The solution is simple: make sure that you have only one call to render or redirect in a single code path.

One thing that can help is and return. Make sure to use and return instead of && return because && return will not work due to the operator precedence in the Ruby Language. This will render a book with special?

Set with the special_show template, while other books will render with the default show template. Another way to handle returning responses to an HTTP request is with redirect_to. As you’ve seen, render tells Rails which view (or other asset) to use in constructing a response.

The redirect_to method does something completely different: it tells the browser to send a new request for a different URL. You can use redirect_to with any arguments that you could use with link_to or url_for. Rails uses HTTP status code 302, a temporary redirect, when you call redirect_to.

Just like the :status option for render, :status for redirect_to accepts both numeric and symbolic header designations. Sometimes inexperienced developers think of redirect_to as a sort of goto command, moving execution from one place to another in your Rails code. This is not correct.

Your code stops running and waits for a new request for the browser.