Understanding redirect_to to render

[nimmolo]

Generally speaking, it seems desirable to convert redirect_to to render.

This is because redirect_to reloads the whole app, but render just continues executing. Until recently it seemed undesirable to attempt this conversion on MO, though, because render doesn't update the browser url by default. But we've recently learned that you can

render("controller/template", location: controller_action_path(:id))

and it will update the browser url.

Note that render generates a different response by the app: it gives a 204 where redirect_to gives 302.
This means:

• assert_redirected_to will not work: it checks for a 300 code and returns false unless the response is actually a redirect. (assert_template can work provisionally, more below.)
• flash will have already been rendered by the time the test checks the response. So assert_flash may not work; instead test for the presence of the flash message in the rendered HTML.

Test expectations have to be adjusted to allow for these differences. But there's another big advantage from converting: Controller tests without assert_redirected_to are a big step closer to integration tests. If we move controller logic to service objects (with their own tests), we will still need to write integration test coverage.

At this point, i maybe can imagine why the Rails maintainers thought it would be no big deal to eliminate controller tests: if they assumed everyone had been keeping up with the redirect/render evolution, then they'd have an easy time with the conversion. As long as controllers are redirecting, though, the test syntax is quite different and it's a daunting change. Converting to render makes the switch easier.

How to proceed?

Big moves

• The first priority may be refactoring controller test methods that hinge on redirects, like requires_login, post_requires_user, etc. Change these to pass kwargs including a redirect/render switch that defaults to redirect, and develop a parallel set of assertions that does not assert redirects, but rather tests for HTML via renders.
• Change ApplicationController methods that redirect, like show_index_of_objects, to render, but wow, lots of index tests will need updating.

Gotchas

• render("template") renders an action's template, it doesn't goto the other action. So you need to set up the same ivars as the action that usually renders the template
• Templates in redirected views must use partial paths relative to the /views directory, or the calling action will look for the partial in its own template directory

Then, we can start converting controllers and tests. Working outline:

• Change the controller action to render, if that's appropriate.
• Change the relevant controller test assertion from assert_redirected_to to assert_template
• Add an HTML ID to a primary div on the template.erb files — these should be unique so they indicate the page template via HTML
• Replace assert_template with an assert_select(“#html_element_id")