ericgray
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.
Marked As Solved
chrismccord
simplified, we generate function heads like this:
def match(conn, "GET", ["users"]), do: UserController.call(conn, UserController.init(:index))
def match(conn, "GET", ["users", id]) do
new_params = Map.merge(conn.params, %{"id" => id})
UserController.call(%{conn | params: new_params}, UserController.init(:show))
end
def match(conn, _, _) do: # raise Phoenix.NotFoundError ...
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blatyo
It delegates part of that responsibility to plug and this function:
https://github.com/elixir-plug/plug/blob/master/lib/plug/router/utils.ex#L52-L67
The primary logic starts here:
https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/router.ex#L321-L324
The thing that generates the final AST is here:
https://github.com/phoenixframework/phoenix/blob/master/lib/phoenix/router.ex#L353-L386
My understanding is that it effectively turns into something like:
def __match_route__(conn, :get, ["users", id], "www.google.com")
That means, when it initially gets a request, it just splits the path by / and can then match on that.
ericgray
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!
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