polyPicker - allows for picking a polymorphic object in ActiveAdmin formtastic with select

_poly_picker.html.slim

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/ This partial is meant to allow for picking an object of polymorphic type, returning both a type and id
/ It is comprised of two select elements, the first of which controls the available choices in the second
/ It relies on the poly_picker.js.coffee script on the front end
/ It expects to find these locals:
/   f: a form builder
/   field: the target field for the picker
/   types: an array of classes which may be selected

/ There must be a collection_action :options defined in ActiveAdmin for each potential type,
/ which returns a collection of options corresponding to that type.
/ Failure to do this will return a method_not_found for the path to this action.
/ A hash of these paths is attached to the source select element

- type_field = "#{field}_type".to_sym
- id_field  = "#{field}_id".to_sym
- unpicked = f.object.send(field).nil? # has this field been picked already?
- empty_message = 'Select a type'

- path_collection = types.inject({}) { |hash, type| hash[type] = polymorphic_path([:options, :admin, type.name.underscore.pluralize]); hash}
- id_collection = unpicked ? [[empty_message, nil]] : Object.const_get(f.object.send(type_field)).all

- f.inputs field, class: 'inputs poly-picker' do
  - f.input type_field, as: :select, collection: types, input_html: {class: 'poly-picker-source', data: {poly_picker_empty: content_tag(:option, empty_message), poly_picker_paths: path_collection}}
  - f.input id_field, as: :select, collection: id_collection, input_html: {class: 'poly-picker-target', disabled: unpicked}

ordinary.rb

include ActionView::Helpers::FormOptionsHelper
ActiveAdmin.register Ordinary do
  collection_action :options, method: :get do
    render text: options_from_collection_for_select(Ordinary.all, :id, :name)
  end
end

poly_picker.js.coffee

#            _      ___ _    _           
#  _ __  ___| |_  _| _ (_)__| |_____ _ _ 
# | '_ \/ _ \ | || |  _/ / _| / / -_) '_|
# | .__/\___/_|\_, |_| |_\__|_\_\___|_|  
# |_|          |__/                 s.c14

# see documentation in poly_picker.html.slim

jQuery ->
  loading_option = ->
    $("<option>").text("Loading...").val null

  $(".poly-picker .poly-picker-source").on 'change', ->
    target = $(this).closest(".poly-picker").find(".poly-picker-target")
    target.attr "disabled", "disabled"
    targetPath = $(this).data('poly-picker-paths')[$(this).val()]
    if targetPath
      target.html(loading_option()).load targetPath, ->
        target.removeAttr "disabled"

    else
      target.html $(this).data("poly-picker-empty")
      # target.removeAttr "disabled"

typical_admin.rb

ActiveAdmin.register Typical do
  form do |f|
    f.template.render partial: 'admin/shared/poly_picker', locals: {f: f, field: :pickee, types: [Ordinary]}
    f.actions
  end
end