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

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:

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.