Refactor and extract DefaultPaginationStrategy.

lib/api/helpers/pagination.rb

 module API
   module Helpers
     module Pagination
+      def paginate(relation)
+        DefaultPaginationStrategy.new(self).paginate(relation)
+      end
+
+      class DefaultPaginationStrategy
+        attr_reader :ctx
+        delegate :params, :header, :request, to: :ctx
+
+        def initialize(ctx)
+          @ctx = ctx
+        end
+
         def paginate(relation)
           relation = add_default_order(relation)
 
@@ -11,6 +23,14 @@ module API
 
         private
 
+        def add_default_order(relation)
+          if relation.is_a?(ActiveRecord::Relation) && relation.order_values.empty?
+            relation = relation.order(:id)
+          end
+
+          relation
+        end
+
         def add_pagination_headers(paginated_data)
           header 'X-Per-Page',    paginated_data.limit_value.to_s
           header 'X-Page',        paginated_data.current_page.to_s
@@ -53,17 +73,10 @@ module API
           [paginated_data.total_pages, 1].max
         end
 
-      def add_default_order(relation)
-        if relation.is_a?(ActiveRecord::Relation) && relation.order_values.empty?
-          relation = relation.order(:id)
-        end
-
-        relation
-      end
-
         def data_without_counts?(paginated_data)
           paginated_data.is_a?(Kaminari::PaginatableWithoutCount)
         end
       end
     end
+  end
 end

lib/gitlab/serializer/pagination.rb

@@ -17,8 +17,6 @@ module Gitlab
         end
       end
 
-      private
-
       # Methods needed by `API::Helpers::Pagination`
       #