Add a InlineBelongsTo field

[rakso-startkoden]

Hi!
I was converting my existing admin to using Backpex and I have cases in my apps that i think is quite common and should be considered.

I have a Schema "A" with a belongs_to to schema "B". But I don't want to have a entire live resource for B since its only ever used in association with A. Which means that I usually just have cast_assoc on schema a and use inputs_for for the nested changeset of B while editing A.

From looking in the source this would be a mix between the InlindeCrud, BelongsTo and the HasManyThrough field currently defined in Backpex. It could behave in a very similar manner in the forms as the HasManyThrough with the modal, or simply have them embedded in some kind of nested box inside the form.

I always think that screenshots for context is helpful so here is an example from my current admin where Schema Service has a belongs_to to a schema called Presentation page, which is in a separate schema because its not always needed in the context of services but still unique for a specific service.

I tried making my own based on the existing field modules. And could modify the inlinecrud to not expect a list of assocs but a single one instead, but was unable to get the rendering of field modules and not only text field working properly.

[rakso-startkoden]

I guess essentially I would like something like this to be possible:

  @impl Backpex.LiveResource
  def fields do
    [
      name: %{
        module: HubbyWeb.CustomFields.TranslatedText,
        searchable: true,
        label: "Name"
      },
      slug: %{
        module: Backpex.Fields.Text,
        only: [:show, :index],
        label: "Slug"
      },
      presentation_page: %{
        module: HubbyWeb.CustomFields.InlineBelongsTo,
        label: "Presentation page",
        type: :assoc,
        display_field: :title,
        except: [:index],
        searchable: false,
        custom_alias: :custom_presentation_page,
        child_fields: [
          title: %{
            module: HubbyWeb.CustomFields.TranslatedText,
            searchable: true,
            label: "Title"
          },
          content: %{
            module: HubbyWeb.CustomFields.TranslatedText,
            searchable: false,
            label: "Content"
          }
        ]
      },

Which I guess would require some kind of mix with code from HasManyThrough to render the child field as correct components?