Optimization of options function in Select / MultiSelect

[dorianamouroux]

Problem

Field options functions (like options in MultiSelect or Select) are called on every form update/event, causing repeated database queries and gives a slow UX:

def fields do
  [
    genre_ids: %{
      module: Backpex.Fields.MultiSelect,
      options: &genre_options/1  # Called on every form update
    }
  ]
end

defp genre_options(assigns) do
  author = author_from_assigns(assigns)
  Genre
  |> where([g], g.id in ^(author.genre_ids || []))
  |> Repo.all()  # ❌ Queries DB on every event
  |> Enum.map(&{"#{&1.name}", &1.id})
end

Use Case

A Book resource has a genre_ids field. Options depend on the associated Author's genre_ids array. Currently, genres are queried on every form update, causing performance issues.

Proposed Solutions

Option 1: Callback to modify assigns after item loads

@impl Backpex.LiveResource
def after_item_loaded(socket, item) do
  genres = load_genres_for_author(item.author)
  assign(socket, :genres, genres)
end

defp genre_options(assigns) do
  genres = Map.get(assigns, :genres, [])
  Enum.map(genres, &{"#{&1.name}", &1.id})
end

Option 2: Allow item_query to return assigns modifications

def item_query(query, _action, _assigns) do
  query = from(b in query, preload: [:author])
  genres = load_genres_for_author(...)
  {query, [genres: genres]}  # Return query + assigns
end

Option 3: Call options function only once

Cache the options result and reuse it on subsequent form updates:

def fields do
  [
    genre_ids: %{
      module: Backpex.Fields.MultiSelect,
      options: &genre_options/1,
      cache_options: true  # NEW: Only call once, cache result
    }
  ]
end

# Called once when item loads, cached for all form updates
defp genre_options(assigns) do
  author = author_from_assigns(assigns)
  Genre
  |> where([g], g.id in ^(author.genre_ids || []))
  |> Repo.all()
  |> Enum.map(&{"#{&1.name}", &1.id})
end

Benefits

• Performance: Avoid repeated database queries on every form update
• Flexibility: Multiple approaches to optimize options functions

---

Would this feature be useful for the Backpex community? I'm happy to look into implementation this if the maintainers are open to it!

[Flo0807]

Hey, thanks for the suggestion!

I see the problem. We have a similar issue with the index view when index_editable is enabled. If selects do not depend on the value of another field, every select has the same options. This results in many identical database queries on page load as each select loads the list of options on their own.

Let me think about the suggested options to address this.