Fix broken Selectize on polymorphic select field (#2839)

When building the select field in the polymorphic form, we omitted to
pass `nil` to the choices argument, which resulted in an erroneous
argument order, and a {} was passed to it instead.
This caused the selectize gem to fail applying the `.selectize` method
under the hood. We introduced this in #2447.

Passing `nil` will now restore its intended behavior and allow selectize
to perform the dynamic seach in the select field again.

To ensure we don't hit this problem again, we implement some feature
tests which specifically trigger the Selectize functionality.

Fixes #2821

Author: heyvaleria

app/views/fields/polymorphic/_form.html.erb

@@ -22,7 +22,7 @@ This partial renders an input element for polymorphic relationships.
 
   <div class="field-unit__field">
     <%= pf.hidden_field(:type, value: field.class.name) %>
-    <%= pf.select(:value, {}, data: {controller: field.html_controller}) do %>
+    <%= pf.select(:value, nil, {}, data: {controller: field.html_controller}) do %>
       <%= grouped_options_for_select(field.associated_resource_grouped_options, field.selected_global_id, prompt: true) %>
     <% end %>
   </div>

spec/features/edit_page_spec.rb

@@ -104,4 +104,18 @@
     customer.reload
     expect(customer.territory).to eq(country)
   end
+
+  it "allows selecting a resource and submitting the form", :js do
+    _blog_post = create(:blog_post, title: "How to Bake Bread")
+    _blog_post2 = create(:blog_post, title: "How to Play Guitar")
+    tag = create(:blog_tag)
+
+    visit edit_admin_blog_tag_path(tag)
+    find(".selectize-input").click
+    find(".selectize-input").fill_in(with: "Bake Bread")
+    find(".option", text: "How to Bake Bread").click
+    click_button "Update Tag"
+
+    expect(page).to have_content("How to Bake Bread")
+  end
 end

spec/features/log_entries_form_spec.rb

@@ -15,6 +15,20 @@
     )
   end
 
+  it "allows to type in the select field to find a match and select it", :js do
+    _customer = create(:customer, name: "Brucey")
+    _customer2 = create(:customer, name: "Nacho")
+
+    visit new_admin_log_entry_path
+    fill_in "Action", with: "create"
+    find(".selectize-input").click
+    find(".selectize-input").fill_in(with: "Brucey")
+    find(".option", text: "Brucey").click
+    click_on "Create Log entry"
+
+    expect(page).to have_content("Brucey")
+  end
+
   it "shows the selected logeable value" do
     customer = create(:customer)
     log_entry = create(:log_entry, logeable: customer)

spec/features/new_page_spec.rb

@@ -0,0 +1,16 @@
+require "rails_helper"
+
+RSpec.describe "Admin creates a new tag", type: :feature, js: true do
+  it "allows selecting a resource and submitting the form" do
+    _blog_post = create(:blog_post, title: "How to Bake Bread")
+    _blog_post2 = create(:blog_post, title: "How to Play Guitar")
+
+    visit "/admin/blog/tags/new"
+    find(".selectize-input").click
+    find(".selectize-input").fill_in(with: "Bake Bread")
+    find(".option", text: "How to Bake Bread").click
+    click_button "Create Tag"
+
+    expect(page).to have_content("How to Bake Bread")
+  end
+end