Move filter attributes retrieval to view

Because of when the user override the `scoped_resource` method in the
controller, the filter attributes retrieval will also be overriden. Thus
making the filter will not be shown in the view. So I move the retrieval
process to the view to make sure the filter will be shown.

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    administrate_filterable (0.0.2)
+    administrate_filterable (0.0.3)
       actionview (>= 5.2.2.1)
       administrate (>= 0.17.0)
       rails (>= 4.2)

app/views/admin/application/_index_filter.html.erb

@@ -1,4 +1,7 @@
-<% if @administrate_filterable_attributes.present? %>
+<% resource_new = resource_class.new %>
+<% filter_attributes = AdministrateFilterable::FiltererService.filter_attributes(@dashboard, resource_new) %>
+
+<% if filter_attributes.present? %>
   <% resource_title = display_resource_name(resource_name) %>
 
   <% # TODO: Improve the toggle button user experience (e.g. add open/close animation, add dynamic open/close title, etc) %>
@@ -13,8 +16,8 @@
       <h2>Filter <%= resource_title %></h2>
     </header>
 
-    <%= form_with(model: [:admin, new_resource], method: :get, html: { class: "form administrate-filterable__form" }) do |f| %>
-      <% @administrate_filterable_attributes.each do |attribute| -%>
+    <%= form_with(model: [:admin, resource_new], method: :get, html: { class: "form administrate-filterable__form" }) do |f| %>
+      <% filter_attributes.each do |attribute| -%>
         <% # TODO: Add capability to customize the filter behavior (e.g. search by exact match, search by partial match, etc just like in the ActiveAdmin filter) %>
         <div class="field-unit field-unit--<%= attribute.html_class %> administrate-filterable__field">
           <%= render_field attribute, f: f %>

lib/administrate_filterable/filterer.rb

@@ -12,11 +12,6 @@ def filterable
         # This is a hack to implement the filter functionality by overriding the scoped_resource method
         # It would be better to implement this as a separate controller action, but I don't have time to explore that right now
         define_method(:scoped_resource) do
-          # TODO: Figure out a better way to pass the filter data to the form
-          # This is a hack to get the filter attributes to show up in the form, but it's not ideal
-          # So I tried to make the variable name as unique as possible to avoid collisions
-          @administrate_filterable_attributes = FiltererService.filter_attributes(dashboard, new_resource)
-
           data = resource_class.all
 
           filter_params = params[resource_name]