1 files changed, 137 insertions, 0 deletions
diff --git a/lib/gitlab/pagination/keyset/simple_order_builder.rb b/lib/gitlab/pagination/keyset/simple_order_builder.rb
new file mode 100644
index 00000000000..5ac5737c3be
--- /dev/null
+++ b/lib/gitlab/pagination/keyset/simple_order_builder.rb
@@ -0,0 +1,137 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Pagination
+    module Keyset
+      # This class transforms the `order()` values from an Activerecord scope into a
+      # Gitlab::Pagination::Keyset::Order instance so the query later can be  used in
+      # keyset pagination.
+      #
+      # Return values:
+      # [transformed_scope, true] # true indicates that the new scope was successfully built
+      # [orginal_scope, false] # false indicates that the order values are not supported in this class
+      class SimpleOrderBuilder
+        def self.build(scope)
+          new(scope: scope).build
+        end
+
+        def initialize(scope:)
+          @scope = scope
+          @order_values = scope.order_values
+          @model_class = scope.model
+          @arel_table = @model_class.arel_table
+          @primary_key = @model_class.primary_key
+        end
+
+        def build
+          order = if order_values.empty?
+                    primary_key_descending_order
+                  elsif ordered_by_primary_key?
+                    primary_key_order
+                  elsif ordered_by_other_column?
+                    column_with_tie_breaker_order
+                  elsif ordered_by_other_column_with_tie_breaker?
+                    tie_breaker_attribute = order_values.second
+
+                    tie_breaker_column_order = Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+                      attribute_name: model_class.primary_key,
+                      order_expression: tie_breaker_attribute
+                    )
+
+                    column_with_tie_breaker_order(tie_breaker_column_order)
+                  end
+
+          order ? [scope.reorder!(order), true] : [scope, false] # [scope, success]
+        end
+
+        private
+
+        attr_reader :scope, :order_values, :model_class, :arel_table, :primary_key
+
+        def primary_key_descending_order
+          Gitlab::Pagination::Keyset::Order.build([
+            Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+              attribute_name: model_class.primary_key,
+              order_expression: arel_table[primary_key].desc
+            )
+          ])
+        end
+
+        def primary_key_order
+          Gitlab::Pagination::Keyset::Order.build([
+            Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+              attribute_name: model_class.primary_key,
+              order_expression: order_values.first
+            )
+          ])
+        end
+
+        def column_with_tie_breaker_order(tie_breaker_column_order = default_tie_breaker_column_order)
+          order_expression = order_values.first
+          attribute_name = order_expression.expr.name
+
+          column_nullable = model_class.columns.find { |column| column.name == attribute_name }.null
+
+          nullable = if column_nullable && order_expression.is_a?(Arel::Nodes::Ascending)
+                       :nulls_last
+                     elsif column_nullable && order_expression.is_a?(Arel::Nodes::Descending)
+                       :nulls_first
+                     else
+                       :not_nullable
+                     end
+
+          Gitlab::Pagination::Keyset::Order.build([
+            Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+              attribute_name: attribute_name,
+              order_expression: order_expression,
+              nullable: nullable,
+              distinct: false
+            ),
+            tie_breaker_column_order
+          ])
+        end
+
+        def ordered_by_primary_key?
+          return unless order_values.one?
+
+          attribute = order_values.first.try(:expr)
+
+          return unless attribute
+
+          arel_table[primary_key].to_s == attribute.to_s
+        end
+
+        def ordered_by_other_column?
+          return unless order_values.one?
+
+          attribute = order_values.first.try(:expr)
+
+          return unless attribute
+          return unless attribute.try(:name)
+
+          model_class.column_names.include?(attribute.name.to_s)
+        end
+
+        def ordered_by_other_column_with_tie_breaker?
+          return unless order_values.size == 2
+
+          attribute = order_values.first.try(:expr)
+          tie_breaker_attribute = order_values.second.try(:expr)
+
+          return unless attribute
+          return unless tie_breaker_attribute
+
+          model_class.column_names.include?(attribute.name.to_s) &&
+            arel_table[primary_key].to_s == tie_breaker_attribute.to_s
+        end
+
+        def default_tie_breaker_column_order
+          Gitlab::Pagination::Keyset::ColumnOrderDefinition.new(
+            attribute_name: model_class.primary_key,
+            order_expression: arel_table[primary_key].desc
+          )
+        end
+      end
+    end
+  end
+end