Deprecate `*_path` methods in mailers

Email does not support relative links since there is no implicit host. Therefore all links inside of emails must be fully qualified URLs. All path helpers are now deprecated. When removed, the error will give early indication to developers to use `*_url` methods instead.

Currently if a developer uses a `*_path` helper, their tests and `mail_view` will not catch the mistake. The only way to see the error is by sending emails in production. Preventing sending out emails with non-working path's is the desired end goal of this PR.

Currently path helpers are mixed-in to controllers (the ActionMailer::Base acts as a controller). All `*_url` and `*_path` helpers are made available through the same module. This PR separates this behavior into two modules so we can extend the `*_path` methods to add a Deprecation to them. Once deprecated we can use this same area to raise a NoMethodError and add an informative message directing the developer to use `*_url` instead.

The module with warnings is only mixed in when a controller returns false from the newly added `supports_relative_path?`.

Paired @sgrif & @schneems

1 files changed, 16 insertions, 0 deletions

diff --git a/guides/source/action_mailer_basics.md b/guides/source/action_mailer_basics.md
index cb1c1c653d..9ad9319255 100644
--- a/guides/source/action_mailer_basics.md
+++ b/guides/source/action_mailer_basics.md
@@ -414,6 +414,22 @@ globally in `config/application.rb`:
 config.action_mailer.default_url_options = { host: 'example.com' }
 ```
 
+Because of this behavior you cannot use any of the `*_path` helpers inside of
+an email. Instead you will need to use the associated `*_url` helper. For example
+instead of using
+
+```
+<%= link_to 'welcome', welcome_path %>
+```
+
+You will need to use:
+
+```
+<%= link_to 'welcome', welcome_url %>
+```
+
+By using the full URL, your links will now work in your emails.
+
 #### generating URLs with `url_for`
 
 You need to pass the `only_path: false` option when using `url_for`. This will