Where to put shared controller code

Question

I have a shared controller function that I'd like to be reused across controllers:

  def render_unprocessable_entity(conn, changeset) do
    conn
    |> put_status(:unprocessable_entity)
    |> render(ExampleApp.ChangesetView, "error.json", changeset: changeset)
  end

Question: where can I put this? I've tried to put it into controllers/helpers/controller_helper.ex and it says: undefined function put_status/2. I can't just add use ExampleApp.Web, :controller in this helper because it will conflict with existing controller. I could use it as regular module and use alias, but that's more typing of ControllerHelper everywhere.

I could put it into web.ex maybe? But maybe I should not make that file too big?

What's the best way to DRY up the code?

Answer 1

Use Kernel.SpecialForms.import/2 to import all exported (public) functions from the desired module (from the helper in this particular case) without explicit namespacing:

controller_helper.ex (or any other module)

defmodule My.ControllerHelper do
  def put_status(param), do: IO.puts(param) # whatever
end

my_controller.ex (or any other module)

import My.ControllerHelper # ⇐ HERE !!!

defmodule My.Controller do
  use Waveia.Web, :controller

  def render_unprocessable_entity(conn, changeset) do
    conn
    |> put_status(:unprocessable_entity) # ⇐ HERE !!!
    |> render(...)
  end
  ...
end

---

Whether you want to have an access from the single module to all those using it, you might override __using__/2 callback to import the module, that is calling use Helper.

The example of this would be:

defmodule ExampleApp.ControllerHelper do
  defmacro __using__(opts) do
    quote do
      defp render_unprocessable_entity(conn, changeset) do
        conn
        |> put_status(:unprocessable_entity)
        |> render(ExampleApp.ChangesetView, "error.json", changeset: changeset)
      end
    end
  end
end

Answer 2

These functions (put_status/2 and render/3) exist in other modules that are imported when you do use Waveia.Web, :controller

You can do:

def render_unprocessable_entity(conn, changeset) do
  conn
  |> Plug.Conn.put_status(:unprocessable_entity)
  |> Phoenix.Controller.render(ExampleApp.ChangesetView, "error.json", changeset: changeset)
end

You could define this in a module then import the module inside the controller function in web.ex, however it should be noted that any time you import functions in to a module, you hide where the module is defined. This means other people working on the code base may have to do some digging round the code to find where the function is defined. Instead, I'd recommend doing:

import MyModule, only: [render_unprocessable_entity: 2]

This way someone viewing the controller code can see exactly where the render_unprocessable_entity/2 function comes from.