Add new_item_builder option to Ecto Adapter

[cervinka]

This change allows you to change the default form prefill when creating a new entry. This is useful, for example, if we want to create an action to copy an item, or create an item based on a template - these cases are very common in administration.

Example of an action to create a copy of an item:

Add a copy action using item_action:

defmodule MyAppWeb.Backpex.Actions.CopyPreparationCategory do
  use BackpexWeb, :item_action

  @impl Backpex.ItemAction
  def icon(assigns, _item) do
    ~H"""
    <Backpex.HTML.CoreComponents.icon name="hero-document-duplicate" class="h-5 w-5 cursor-pointer transition duration-75 hover:scale-110 hover:text-blue-600" />
    """
  end

  @impl Backpex.ItemAction
  def label(_assigns, _item), do: Backpex.translate("Copy")

  @impl Backpex.ItemAction
  def handle(socket, [item | _items], _data) do
    path =
      Router.get_path(socket, socket.assigns.live_resource, socket.assigns.params, :new, %{
        copy_from: item.id
      })

    {:ok, Phoenix.LiveView.push_patch(socket, to: path)}
  end
end

Add action to LiveResource:

  @impl Backpex.LiveResource
  def item_actions(default_actions) do
    [
      {:copy, %{module: MyAppWeb.Backpex.Actions.CopyPreparationCategory, only: [:row]}}
      | default_actions
    ]
  end

Up to now, everything can be done with standard Backpex tools. But we need to create an item with pre-filled values. This is not possible now, but modifying to multiple lines would allow such flexibility.

I suggest to add new optional adapter option new_item_builder , its use would then look like this:

use Backpex.LiveResource,
    adapter_config: [
      ...
      new_item_builder: &__MODULE__.builder/1 # new option
    ],
    ....

  def builder(socket) do
    # we have access to all parameters here
    source_id = socket.assigns.params["copy_from"]

    if source_id do
      source = Preparations.get_preparation_category!(source_id)
      %PreparationCategory{name: source.name, position: source.position}
    else
      %PreparationCategory{}
    end
  end

The implementation is very straightforward. Instead of the existing code for creating new items:

# lib/backpex/live_resource.ex
    empty_item = schema.__struct__()

use this

    new_item = if (builder = live_resource.config(:adapter_config)[:new_item_builder]),
      do: builder.(socket),
      else: schema.__struct__()

I think this small change would increase the possibilities that Backpex offers.