reduce count calls in collection representer

will_paginated, when actually fetching the records will do an SQL count on top of the fetching. We don`t need that
in our offset paginated represters since those implement their own counts.

And if the per_page is 0, we don`t even have to try to fetch any objects so we can use `none` to avoid the database
roundtrip.

lib/api/decorators/offset_paginated_collection.rb

@@ -107,9 +107,20 @@ def query_params(page = @page, page_size = @per_page)
       end
 
       def paged_models(models)
-        # FIXME: calling :to_a is a hack to circumvent a counting error in will_paginate
-        # see https://github.com/mislav/will_paginate/issues/449
-        models.page(@page).per_page(@per_page).to_a
+        if @per_page == 0
+          # Optimization. If we are not interested in the model, we can
+          # save the round trip to the database.
+          models.none
+        else
+          # Using WillPaginate as we have used it before but avoid the builtin
+          # page(@page).per_page(@per_page) way of fetching
+          # as it will, on top of fetching the values, also do a count of all elements matching the query
+          # which we do not need at this place.
+          page_number = ::WillPaginate::PageNumber(@page.nil? ? 1 : @page)
+          models
+            .offset(page_number.to_offset(@per_page).to_i)
+            .limit(@per_page)
+        end
       end
     end
   end

lib/api/v3/work_packages/work_package_collection_representer.rb

@@ -191,12 +191,11 @@ def schema_pairs
         def all_cfs_of_project
           @all_cfs_of_project ||= represented
                                   .group_by(&:project_id)
-                                  .map { |id, wps| [id, wps.map(&:available_custom_fields).flatten.uniq] }
-                                  .to_h
+                                  .transform_values { |wps| wps.map(&:available_custom_fields).flatten.uniq }
         end
 
         def paged_models(models)
-          models.page(@page).per_page(@per_page).pluck(:id)
+          super.pluck(:id)
         end
 
         def _type

spec/lib/api/v3/groups/group_collection_representer_spec.rb

@@ -33,13 +33,13 @@
   let(:groups) do
     build_stubbed_list(:group, 3).tap do |groups|
       allow(groups)
-        .to receive(:per_page)
-        .with(page_size)
+        .to receive(:offset)
+        .with(page - 1)
         .and_return(groups)
 
       allow(groups)
-        .to receive(:page)
-        .with(page)
+        .to receive(:limit)
+        .with(page_size)
         .and_return(groups)
 
       allow(groups)

spec/lib/api/v3/memberships/membership_collection_representer_spec.rb

@@ -33,13 +33,13 @@
   let(:members) do
     build_stubbed_list(:member, 3).tap do |members|
       allow(members)
-        .to receive(:per_page)
+        .to receive(:limit)
         .with(page_size)
         .and_return(members)
 
       allow(members)
-        .to receive(:page)
-        .with(page)
+        .to receive(:offset)
+        .with(page - 1)
         .and_return(members)
 
       allow(members)

spec/lib/api/v3/notifications/notification_collection_representer_spec.rb

@@ -35,13 +35,13 @@
     build_stubbed_list(:notification,
                        3).tap do |items|
       allow(items)
-        .to receive(:per_page)
+        .to receive(:limit)
               .with(page_size)
               .and_return(items)
 
       allow(items)
-        .to receive(:page)
-              .with(page)
+        .to receive(:offset)
+              .with(page - 1)
               .and_return(items)
 
       allow(items)

spec/lib/api/v3/placeholder_users/placeholder_user_collection_representer_spec.rb

@@ -41,13 +41,13 @@
     placeholders = build_stubbed_list(:placeholder_user,
                                       actual_count)
     allow(placeholders)
-      .to receive(:per_page)
+      .to receive(:limit)
       .with(page_size)
       .and_return(placeholders)
 
     allow(placeholders)
-      .to receive(:page)
-      .with(page)
+      .to receive(:offset)
+      .with(page - 1)
       .and_return(placeholders)
 
     allow(placeholders)

spec/lib/api/v3/relations/relation_paginated_collection_representer_spec.rb

@@ -36,13 +36,13 @@
   let(:relations) do
     build_stubbed_list(:relation, total).tap do |relations|
       allow(relations)
-        .to receive(:per_page)
+        .to receive(:limit)
               .with(page_size)
               .and_return(relations)
 
       allow(relations)
-        .to receive(:page)
-              .with(page)
+        .to receive(:offset)
+              .with(page - 1)
               .and_return(relations)
 
       allow(relations)

spec/lib/api/v3/users/user_collection_representer_spec.rb

@@ -40,13 +40,13 @@
     users = build_stubbed_list(:user,
                                actual_count)
     allow(users)
-      .to receive(:per_page)
+      .to receive(:limit)
       .with(page_size)
       .and_return(users)
 
     allow(users)
-      .to receive(:page)
-      .with(page)
+      .to receive(:offset)
+      .with(page - 1)
       .and_return(users)
 
     allow(users)