1 files changed, 67 insertions, 0 deletions
diff --git a/lib/gitlab/graphql/pagination/keyset/query_builder.rb b/lib/gitlab/graphql/pagination/keyset/query_builder.rb
new file mode 100644
index 00000000000..331981ce723
--- /dev/null
+++ b/lib/gitlab/graphql/pagination/keyset/query_builder.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Graphql
+    module Pagination
+      module Keyset
+        class QueryBuilder
+          def initialize(arel_table, order_list, decoded_cursor, before_or_after)
+            @arel_table, @order_list, @decoded_cursor, @before_or_after = arel_table, order_list, decoded_cursor, before_or_after
+
+            if order_list.empty?
+              raise ArgumentError.new('No ordering scopes have been supplied')
+            end
+          end
+
+          # Based on whether the main field we're ordering on is NULL in the
+          # cursor, we can more easily target our query condition.
+          # We assume that the last ordering field is unique, meaning
+          # it will not contain NULLs.
+          # We currently only support two ordering fields.
+          #
+          # Example of the conditions for
+          #   relation: Issue.order(relative_position: :asc).order(id: :asc)
+          #   after cursor: relative_position: 1500, id: 500
+          #
+          #   when cursor[relative_position] is not NULL
+          #
+          #       ("issues"."relative_position" > 1500)
+          #       OR (
+          #         "issues"."relative_position" = 1500
+          #         AND
+          #         "issues"."id" > 500
+          #       )
+          #       OR ("issues"."relative_position" IS NULL)
+          #
+          #   when cursor[relative_position] is NULL
+          #
+          #       "issues"."relative_position" IS NULL
+          #       AND
+          #       "issues"."id" > 500
+          #
+          def conditions
+            attr_values = order_list.map { |field| decoded_cursor[field.attribute_name] }
+
+            if order_list.count == 1 && attr_values.first.nil?
+              raise Gitlab::Graphql::Errors::ArgumentError.new('Before/after cursor invalid: `nil` was provided as only sortable value')
+            end
+
+            if order_list.count == 1 || attr_values.first.present?
+              Keyset::Conditions::NotNullCondition.new(arel_table, order_list, attr_values, operators, before_or_after).build
+            else
+              Keyset::Conditions::NullCondition.new(arel_table, order_list, attr_values, operators, before_or_after).build
+            end
+          end
+
+          private
+
+          attr_reader :arel_table, :order_list, :decoded_cursor, :before_or_after
+
+          def operators
+            order_list.map { |field| field.operator_for(before_or_after) }
+          end
+        end
+      end
+    end
+  end
+end