Add Define Conditional Routing Logic In Routes File as a Rails TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1799 TILs and counting..._
+_1800 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1148,6 +1148,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Customize Paths And Helpers For Devise Routes](rails/customize-paths-and-helpers-for-devise-routes.md)
 - [Customize Template For New Schema Migration](rails/customize-template-for-new-schema-migration.md)
 - [Customize The Path Of A Resource Route](rails/customize-the-path-of-a-resource-route.md)
+- [Define Conditional Routing Logic In Routes File](rails/define-conditional-routing-logic-in-routes-file.md)
 - [Define The Root Path For The App](rails/define-the-root-path-for-the-app.md)
 - [Delete Paranoid Records](rails/delete-paranoid-records.md)
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)

rails/define-conditional-routing-logic-in-routes-file.md

@@ -0,0 +1,37 @@
+# Define Conditional Routing Logic In Routes File
+
+I ran into a situation recently where I needed to intercept the behavior a
+common public-facing route in an app. Broadly, the route is for company specific
+rental pages with query parameters that correspond to their available inventory.
+
+What I needed was a way to display a demo version of that rental page ignoring
+everything else about how the request would otherwise be processed, validated,
+and rendered.
+
+Instead of introducing a bunch of weird conditional logic into this already
+complex rental controller, I was able to intercept the request at the routing
+layer when `demo=true` is set and send it to a different controller.
+
+Here is what that section of `config/routes.rb` looks like:
+
+```ruby
+get "rentals/new", to: "rental_demos#show",
+  as: :rental_demo,
+  constraints: ->(request) { request.params[:demo] == "true" }
+
+resources :rentals, only: %i[new create] do
+  # ...
+end
+```
+
+This specifies a `constraint` on the `get` handler matching for a given request.
+If the constraint isn't met, then the route handling logic proceeds where it
+will instead find a match with the original new rentals resource routing.
+
+Now I can reference a version of this URL that includes `demo=true` as a way of
+having an always-available realistic-looking version of the rental page even if
+one of these companies doesn't actively have available inventory.
+
+Those requests will get intercepted by the first matching route handler which
+will send them to the `RentalDemosController` instead of the
+`RentalsController`.