Add a cursor to go to the first page (#42)

* Add a cursor to go to the first page

* small changes

* add a line in CHANGELOG

* add specs for mongo

* Update lib/mongoid/criteria/scrollable/iterator.rb

Co-authored-by: Daniel (dB.) Doubrovkine <[email protected]>

* minor changes

---------

Co-authored-by: Daniel (dB.) Doubrovkine <[email protected]>

CHANGELOG.md

@@ -1,6 +1,7 @@
 ### 2.0.0 (Next)
 
-* [#38](https://github.com/mongoid/mongoid-scroll/pull/38): Allow to reverse the scroll - [@GCorbel](https://github.com/GCorbel).
+* [#38](https://github.com/mongoid/mongoid-scroll/pull/38): Add `previous_cursor` - [@GCorbel](https://github.com/GCorbel).
+* [#42](https://github.com/mongoid/mongoid-scroll/pull/42): Add `first_cursor` - [@GCorbel](https://github.com/GCorbel).
 * Your contribution here.
 
 ### 1.0.1 (2023/03/15)

README.md

@@ -95,6 +95,8 @@ Feed::Item.desc(:position).limit(5).scroll(saved_iterator.previous_cursor) do |r
 end
 ```
 
+Use `saved_iterator.first_cursor` to loop over the first records.
+
 The iteration finishes when no more records are available. You can also finish iterating over the remaining records by omitting the query limit.
 
 ```ruby

lib/mongoid/criteria/scrollable/iterator.rb

@@ -8,6 +8,10 @@ def initialize(previous_cursor:, next_cursor:)
           @previous_cursor = previous_cursor
           @next_cursor = next_cursor
         end
+
+        def first_cursor
+          @first_cursor ||= next_cursor.class.new(nil, next_cursor.sort_options)
+        end
       end
     end
   end

spec/mongo/collection_view_spec.rb

@@ -127,6 +127,17 @@
                 expect(Mongoid.default_client['feed_items'].find.sort(field_name => 1).limit(2).scroll(second_iterator.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(third_iterator.previous_cursor, field_type: field_type).to_a).to eq(records.skip(2).limit(2).to_a)
               end
+              it 'can loop over the first records with the first cursor' do
+                first_cursor = nil
+                records = Mongoid.default_client['feed_items'].find.sort(field_name => 1)
+                cursor = cursor_type.from_record records.skip(4).first, field_name: field_name, field_type: field_type, include_current: true
+
+                records.limit(2).scroll(cursor, field_type: field_type) do |_, iterator|
+                  first_cursor = iterator.first_cursor
+                end
+
+                expect(records.limit(2).scroll(first_cursor, field_type: field_type).to_a).to eq(records.limit(2).to_a)
+              end
             end
           end
         end

spec/mongoid/criteria_spec.rb

@@ -181,6 +181,15 @@
               expect(Feed::Item.asc(field_name).limit(2).scroll(second_iterator.previous_cursor)).to eq(records.limit(2))
               expect(Feed::Item.asc(field_name).limit(2).scroll(third_iterator.previous_cursor)).to eq(records.skip(2).limit(2))
             end
+            it 'can loop over the first records with the first page cursor' do
+              first_cursor = nil
+
+              Feed::Item.asc(field_name).limit(2).scroll(cursor_type) do |_, it|
+                first_cursor = it.first_cursor
+              end
+
+              expect(Feed::Item.asc(field_name).limit(2).scroll(first_cursor).to_a).to eq(Feed::Item.asc(field_name).limit(2).to_a)
+            end
           end
         end
       end