Provide a way to clear belongs_to selection without keyboard

Closes #2090

Author: mshibuya

app/assets/javascripts/rails_admin/ra.filtering-select.js

@@ -85,7 +85,20 @@
         }
       };
 
-      return $.map(
+      if (request.term.length > 0 || this.input.attr('required')) {
+        var additionalOptions = [];
+      } else {
+        var additionalOptions = [{
+          html: $('<span style="color: #888"></span>').append(
+              '<i class="icon-remove"></i> ' +
+              $('<span></span>').text(RailsAdmin.I18n.t("clear")).html()
+          ),
+          value: null,
+          id: null
+        }];
+      }
+
+      return additionalOptions.concat($.map(
         data,
         function(el) {
           var id = el.id || el.value;
@@ -99,7 +112,7 @@
               id: id
             };
           }
-      });
+      }));
     },
 
     _getSourceFunction: function(source) {

config/locales/rails_admin.en.yml

@@ -19,6 +19,7 @@ en:
       ends_with: Ends with
       too_many_objects: "Too many objects, use search box above"
       no_objects: "No objects found"
+      clear: Clear
     loading: "Loading..."
     toggle_navigation: Toggle navigation
     home:

spec/integration/widgets/filtering_select_spec.rb

@@ -44,9 +44,8 @@
   end
 
   context 'on update' do
-    before { visit edit_path(model_name: 'player', id: player.id) }
-
     it 'changes the selected value' do
+      visit edit_path(model_name: 'player', id: player.id)
       expect(find('#player_team_id', visible: false).value).to eq teams[0].id.to_s
       find('input.ra-filtering-select-input').set('Tex')
       page.execute_script("$('input.ra-filtering-select-input').trigger('focus').trigger('keydown')")
@@ -56,11 +55,33 @@
       expect(find('#player_team_id', visible: false).value).to eq teams[1].id.to_s
     end
 
-    it 'clears the current selection' do
+    it 'clears the current selection with making the search box empty' do
+      visit edit_path(model_name: 'player', id: player.id)
       find('input.ra-filtering-select-input').set('')
       page.execute_script("$('input.ra-filtering-select-input').trigger('keyup')")
       expect(find('#player_team_id', visible: false).value).to be_empty
     end
+
+    it 'clears the current selection with selecting the clear option' do
+      visit edit_path(model_name: 'player', id: player.id)
+      within('.filtering-select') { find('.dropdown-toggle').click }
+      find('a.ui-menu-item-wrapper', text: /Clear/).click
+      expect(find('#player_team_id', visible: false).value).to be_empty
+    end
+
+    context 'when the field is required' do
+      before do
+        RailsAdmin.config Player do
+          field(:team) { required true }
+        end
+        visit edit_path(model_name: 'player', id: player.id)
+      end
+
+      it 'does not show the clear option' do
+        within('.filtering-select') { find('.dropdown-toggle').click }
+        is_expected.not_to have_css('a.ui-menu-item-wrapper', text: /Clear/)
+      end
+    end
   end
 
   it 'prevents duplication when using browser back and forward' do