domain.ex

defmodule Myapp.Domain do
  import Plug.Conn
  import Ecto.Model

  @doc false
  def init(opts), do: opts

  @doc false
  def call(conn, _opts) do
    IO.inspect conn
    site_obj = Myapp.Repo.get_by(Myapp.Site, domain: conn.host)
    conn
      |> put_private(:site_obj, site_obj)
  end
end

endpoint.ex

defmodule Myapp.Endpoint do
  ...
  ...
  plug Myapp.Domain
  plug Myapp.Router
end

registration_controller.ex

defmodule Myapp.RegistrationController do
  use Myapp.Web, :controller
  alias Myapp.User
  alias Myapp.UserSite

  # plug :scrub_params, "user" when action in [:create]

  def new(conn, _params) do
    changeset = User.changeset(%User{})
    render conn, changeset: changeset
  end

  def create(conn, %{"user" => user_params}) do
    user_changeset = User.changeset(%User{}, user_params)
    if user_changeset.valid? do
      Repo.transaction fn ->
        user = Repo.insert!(user_changeset)
        user_site = Ecto.Model.build(user, :user_sites, site: site_id(conn))
        Repo.insert!(user_site)

        conn
          |> put_flash(:info, "Your account was created")
          |> put_session(:current_user, user)
          |> redirect(to: "/")
      end
    else
      conn
        |> render("new.html", changeset: user_changeset)
    end
  end
end

registration_controller_test.ex

defmodule Myapp.RegistrationControllerTest do
  use Myapp.ConnCase
  alias Myapp.Site

  @valid_attrs %{email: "a@a.com", passwd: "Abcdef123", passwd_confirmation: "Abcdef123"}
  @invalid_attrs %{}

  setup do
    Repo.insert!(%Site{name: "Abc", slug: "abc", domain: "abc.com", license: "123", address: "Abc", email: "abc@abc.com", phone1: "123-456-7890"})
    conn = %{conn() | host: "abc.com"}
    {:ok, conn: conn}
  end

  test "creates resource and redirects when data is valid", %{conn: conn} do
    # conn = post conn(), "abc.com/register", user: @valid_attrs
    conn = post conn, registration_path(conn, :create), user: @valid_attrs
    assert redirected_to(conn) == "/"
    # assert Repo.get_by(User, @valid_attrs)
  end
end