Remove the selects when counting the last page

The last page of the first collection is only loaded into memory when
it is being viewed. If it isn't loaded into memory, the `#size` call
triggers a count query.

This `#count` would generate an invalid query if our custom preloaded
counts are included by adding a separate `as count_column` alias on
top of the count aliases. Removing the selects in this case will make
sure a valid `COUNT(*)` is generated.

changelogs/unreleased/bvl-fix-count-with-selects.yml

+---
+title: Fix crash when navigating to second page of the group dashbaord when there
+  are projects and groups on the first page
+merge_request: 15456
+author:
+type: fixed

lib/gitlab/multi_collection_paginator.rb

@@ -55,7 +55,9 @@ module Gitlab
     def first_collection_last_page_size
       return @first_collection_last_page_size if defined?(@first_collection_last_page_size)
 
-      @first_collection_last_page_size = paginated_first_collection(first_collection_page_count).count
+      @first_collection_last_page_size = paginated_first_collection(first_collection_page_count)
+                                           .except(:select)
+                                           .size
     end
   end
 end

spec/controllers/groups/children_controller_spec.rb

@@ -280,6 +280,17 @@ describe Groups::ChildrenController do
 
           expect(assigns(:children)).to contain_exactly(other_subgroup, *next_page_projects.take(per_page - 1))
         end
+
+        context 'with a mixed first page' do
+          let!(:first_page_subgroups) { [create(:group,  :public,  parent: group)] }
+          let!(:first_page_projects) { create_list(:project, per_page, :public, namespace: group) }
+
+          it 'correctly calculates the counts' do
+            get :index, group_id: group.to_param, sort: 'id_asc', page: 2, format: :json
+
+            expect(response).to have_gitlab_http_status(200)
+          end
+        end
       end
     end
   end