How does Phoenix pattern matching work on Dynamic Routes

I never fully understood how Phoenix is able to route urls with params like "users/1" to the correct Controller, Action. From what I understand Phoenix pattern matches the path on incoming requests. For example, its easy to see how "/users" will match this get macro.

get "/users", UserController, :index

But how does Phoenix route "/users/1" to this.

get "/users/:id", UserController, :show  

How does Phoenix match "/users/1" to "/users/:id"
I know it can’t be traditional pattern matching otherwise you’d get a no function clause matching error.

There must be some kind of macro magic happening behind the scenes that I don’t understand.


It delegates part of that responsibility to plug and this function:

The primary logic starts here:

The thing that generates the final AST is here:

My understanding is that it effectively turns into something like:

def __match_route__(conn, :get, ["users", id], "")

That means, when it initially gets a request, it just splits the path by / and can then match on that.


Thanks, I was hoping I wouldn’t have to take a deep dive into the source but in order to understand what’s going on under the hood looks like I have no choice.

simplified, we generate function heads like this:

def match(conn, "GET", ["users"]), do:, UserController.init(:index))
def match(conn, "GET", ["users", id]) do
  new_params = Map.merge(conn.params, %{"id" => id}){conn | params: new_params}, UserController.init(:show))
def match(conn, _, _) do: # raise Phoenix.NotFoundError ...

Hey Chris thanks, that really pointed me in the right direction. I dug a little deeper and found this video Erlang Factory 2014 – Write Less, Do More (and Have Fun!) with Elixir Macros The video is a gem and really helped me understand Macros.

I was always curios how the Router layer worked in Phoenix. For any one interested about 31:22 Chris explains how routes work. I also purchased Metaprogramming Elixir, used my Elixir Forum discount and got it for 7 bucks. What a steal!