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

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

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+# 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`.