keep the ordering when fetching previous records

Author: GCorbel

lib/mongo/scrollable.rb

@@ -16,21 +16,37 @@ def scroll(cursor_or_type = nil, options = nil, &_block)
       cursor_options = { field_name: scroll_field, direction: scroll_direction }.merge(options)
       cursor = cursor && cursor.is_a?(cursor_type) ? cursor : cursor_type.new(cursor, cursor_options)
       raise_mismatched_sort_fields_error!(cursor, cursor_options) if different_sort_fields?(cursor, cursor_options)
-      # make a view
-      view = Mongo::Collection::View.new(
-        view.collection,
-        view.selector.merge(cursor.criteria),
-        sort: (view.sort || {}).merge(_id: scroll_direction),
-        skip: skip,
-        limit: limit
-      )
+
+      records = nil
+      if cursor.previous && limit
+        # scroll backards by reversing the sort order, limit and then reverse again
+        pipeline = [
+          { '$match' => view.selector.merge(cursor.criteria) },
+          { '$sort' => { scroll_field => -scroll_direction } },
+          { '$limit' => limit },
+          { '$sort' => { scroll_field => scroll_direction } }
+        ]
+        aggregation_options = view.options.except(:sort)
+        records = view.aggregate(pipeline, aggregation_options)
+      else
+        # make a view
+        records = Mongo::Collection::View.new(
+          view.collection,
+          view.selector.merge(cursor.criteria),
+          sort: (view.sort || {}).merge(_id: scroll_direction),
+          skip: skip,
+          limit: limit
+        )
+      end
       # scroll
       if block_given?
-        view.each do |record|
-          yield record, cursor_type.from_record(record, cursor_options), cursor_type.from_record(record, cursor_options.merge(previous: true))
+        previous_cursor = nil
+        records.each do |record|
+          previous_cursor ||= cursor_type.from_record(record, cursor_options.merge(previous: true))
+          yield record, cursor_type.from_record(record, cursor_options), previous_cursor
         end
       else
-        view
+        records
       end
     end
   end

lib/mongoid/criteria/scrollable.rb

@@ -14,9 +14,10 @@ def scroll(cursor_or_type = nil, &_block)
         raise_mismatched_sort_fields_error!(cursor, cursor_options) if different_sort_fields?(cursor, cursor_options)
         records = find_records(criteria, cursor)
         if block_given?
-          first_cursor = cursor_from_record(cursor_type, records.first, cursor_options.merge(previous: true))
+          previous_cursor = nil
           records.each do |record|
-            yield record, cursor_from_record(cursor_type, record, cursor_options), first_cursor
+            previous_cursor ||= cursor_from_record(cursor_type, record, cursor_options.merge(previous: true))
+            yield record, cursor_from_record(cursor_type, record, cursor_options), previous_cursor
           end
         else
           records

spec/mongo/collection_view_spec.rb

@@ -107,26 +107,26 @@
                 expect(cursor.tiebreak_id).to eq record['_id']
               end
               it 'can scroll back with the previous cursor' do
-                records = []
-                first_cursor = nil
-                second_cursor = nil
-                third_cursor = nil
-                criteria = Mongoid.default_client['feed_items'].find.limit(2)
-                criteria.scroll(cursor_type) do |record, next_cursor|
-                  second_cursor = next_cursor
+                cursor = nil
+                first_previous_cursor = nil
+                second_previous_cursor = nil
+
+                Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(cursor_type, field_type: field_type) do |_, next_cursor|
+                  cursor = next_cursor
                 end
-                criteria.scroll(second_cursor) do |record, next_cursor, previous_cursor|
-                  first_cursor = previous_cursor
-                  third_cursor = next_cursor
+
+                Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(cursor, field_type: field_type) do |_, next_cursor, previous_cursor|
+                  cursor = next_cursor
+                  first_previous_cursor = previous_cursor
                 end
 
-                first_item = Mongoid.default_client['feed_items'].find.to_a[0]
-                from_item = Mongoid.default_client['feed_items'].find.scroll(first_cursor).to_a.first
-                expect(from_item).to eq first_item
+                Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(cursor, field_type: field_type) do |_, _, previous_cursor|
+                  second_previous_cursor = previous_cursor
+                end
 
-                fifth_item = Mongoid.default_client['feed_items'].find.to_a[4]
-                from_item = Mongoid.default_client['feed_items'].find.scroll(third_cursor).to_a.first
-                expect(from_item).to eq fifth_item
+                records = Mongoid.default_client['feed_items'].find.sort(field_name => 1)
+                expect(Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(first_previous_cursor, field_type: field_type).to_a).to eq(records.limit(2).to_a)
+                expect(Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(second_previous_cursor, field_type: field_type).to_a).to eq(records.skip(2).limit(2).to_a)
               end
             end
           end