Fix problem with sorting by case in GraphQL

lib/gitlab/graphql/pagination/keyset/order_info.rb

@@ -94,6 +94,8 @@ module Gitlab
               [order_value.expr.expressions[0].name.to_s, order_value.direction, order_value.expr]
             elsif ordering_by_similarity?(order_value)
               ['similarity', order_value.direction, order_value.expr]
+            elsif ordering_by_case?(order_value)
+              [order_value.expr.case.name.to_s, order_value.direction, order_value.expr]
             else
               [order_value.expr.name, order_value.direction, nil]
             end
@@ -108,6 +110,11 @@ module Gitlab
           def ordering_by_similarity?(order_value)
             Gitlab::Database::SimilarityScore.order_by_similarity?(order_value)
           end
+
+          # determine if ordering using CASE
+          def ordering_by_case?(order_value)
+            order_value.expr.is_a?(Arel::Nodes::Case)
+          end
         end
       end
     end

spec/lib/gitlab/graphql/pagination/keyset/order_info_spec.rb

@@ -63,6 +63,17 @@ RSpec.describe Gitlab::Graphql::Pagination::Keyset::OrderInfo do
         expect(order_list.first.sort_direction).to eq :desc
       end
     end
+
+    context 'when ordering by CASE', :aggregate_failuers do
+      let(:relation) { Project.order(Arel::Nodes::Case.new(Project.arel_table[:pending_delete]).when(true).then(100).else(1000).asc) }
+
+      it 'assigns the right attribute name, named function, and direction' do
+        expect(order_list.count).to eq 1
+        expect(order_list.first.attribute_name).to eq 'pending_delete'
+        expect(order_list.first.named_function).to be_kind_of(Arel::Nodes::Case)
+        expect(order_list.first.sort_direction).to eq :asc
+      end
+    end
   end
 
   describe '#validate_ordering' do