Dynamic enum field values in nested association forms in Rails Admin

change.rb

# app/models/change.rb
class Change < ActiveRecord::Base
  belongs_to :changeset, inverse_of: :changed_fields
  validates :changeset, presence: true, associated: true
  validates :field, presence: true, uniqueness: {scope: :changeset_id}, inclusion: { in: :field_enum }

  def field_enum
    if changeset.try(:model).present?
      changeset.model_attributes
    else
      all_attributes
    end
  end

  private
  
  def all_attributes
    ::MODELS.collect{|k,h| h["attributes"]}.flatten.uniq.sort
  end
end

changes_controller.rb

# app/controllers/changes_controller.rb
class ChangesController < ApplicationController
  before_filter :authenticate_user!
  
  def get_attributes_for
    attributes = Change.new(changeset: Changeset.new(model: params[:model])).field_enum
    render json: attributes
  end
end

changeset.rb

# app/models/changeset.rb
class Changeset < ActiveRecord::Base
  has_many :changed_fields, class_name: "Change", dependent: :destroy, inverse_of: :changeset
  accepts_nested_attributes_for :changed_fields, :allow_destroy => true
  validates :model, presence: true, inclusion: { in: :model_enum }

  def model_enum
    ::MODELS.keys.sort
  end
  
  def model_attributes
    ::MODELS[model].try(:[], "attributes").try(:sort) || []
  end
end

routes.rb

# config/routes.rb
Rails.application.routes.draw do
  # ...
  get 'changes/attributes_for/:model' => 'changes#get_attributes_for'
  # ...
end

ui.coffee

# app/assets/javascripts/rails_admin/custom/ui.coffee
$ = jQuery
updateAttributes = (selects = $("select[id^=changeset_changed_fields_attributes]")) ->
  model = $('#changeset_model').val()
  $.get("/changes/attributes_for/" + model).done (values) ->
    elements = []
    $.each values, (idx, value) ->
      elements.push($("<option></option>").attr("value", value).text(value))
    selects.each ->
      select = $(this)
      selected = select.val()
      select.filteringSelect("destroy").find("option:gt(0)").remove().end().append(elements).filteringSelect().val(select)
  
$(document).on 'change', '#changeset_model', (event) ->
  updateAttributes()
  
$(document).on 'nested:fieldAdded', 'form', (content) ->
  updateAttributes(content.field.find("select[id^=changeset_changed_fields_attributes]"))