The session must be used sparingly. Getting the data from conn.private and putting it into session before the response is sent is specific to the built-in Flash functionality. It’s not part of the general request lifecycle; it would be terrible if it were.
fetch_flash/2 returns a register_before_send/2 function, which is a callback that gets called before the response is sent. And from there the data from conn.private is stored in session.
After some research, a simple solution using the built-in Flash mechanism to implement full-PRG (so including redirecting after invalid form submission), are the following steps:
- Invalid form is submitted,
create_*action is called - Submitted form parameters are put into Flash, if changeset is invalid
- Redirect to the
new_*action - Rebuild the changeset based on parameters that were stored in Flash (if present)
- Form displays all errors
Sample code (example is a form allowing to add/edit/remove items for a user):
def new_user_items(conn, %{"user_id" => user_id}) do
changeset =
Business.get_user!(user_id, preload: [:items])
|> Business.change_user(get_flash(conn, :create_user_items_params) || %{}) # <---
|> Map.put(:action, :update)
render(conn, "new_user_items.html", changeset: changeset, user_id: user_id)
end
def create_user_items(conn, %{"user_id" => user_id, "user" => user_params}) do
user = Business.get_user!(user_id)
case Business.add_items(user, user_params) do
{:ok, _} ->
redirect(conn, to: Routes.registration_path(conn, :next_action, user_id))
{:error, %Ecto.Changeset{} = changeset} ->
conn
|> put_flash(:create_user_items_params, user_params) # <---
|> redirect(to: Routes.registration_path(conn, :new_user_items, user_id)) # <---
end
end
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