Gives the previous cursor in the scroll block

CHANGELOG.md

@@ -1,5 +1,6 @@
-### 1.0.2 (Next)
+### 1.1.0 (Next)
 
+* [#38](https://github.com/mongoid/mongoid-scroll/pull/38): Allow to reverse the scroll - [@GCorbel](https://github.com/GCorbel).
 * Your contribution here.
 
 ### 1.0.1 (2023/03/15)

README.md

@@ -76,7 +76,16 @@ Feed::Item.desc(:position).limit(5).scroll do |record, next_cursor|
 end
 ```
 
-Resume iterating using the previously saved cursor.
+Resume iterating using saved cursor and save the cursor to go backward.
+
+```ruby
+Feed::Item.desc(:position).limit(5).scroll(saved_cursor) do |record, _, previous_cursor|
+  # each record, one-by-one
+  saved_cursor = previous_cursor
+end
+```
+
+Loop over the first records again.
 
 ```ruby
 Feed::Item.desc(:position).limit(5).scroll(saved_cursor) do |record, next_cursor|
@@ -179,7 +188,7 @@ Feed::Item.desc(:created_at).scroll(cursor) # Raises a Mongoid::Scroll::Errors::
 
 ### Standard Cursor
 
-The `Mongoid::Scroll::Cursor` encodes a value and a tiebreak ID separated by `:`, and does not include other options, such as scroll direction. Take extra care not to pass a cursor into a scroll with different options. 
+The `Mongoid::Scroll::Cursor` encodes a value and a tiebreak ID separated by `:`, and does not include other options, such as scroll direction. Take extra care not to pass a cursor into a scroll with different options.
 
 ### Base64 Encoded Cursor
 

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 backwards 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)
+        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

@@ -12,13 +12,15 @@ def scroll(cursor_or_type = nil, &_block)
         cursor_options = build_cursor_options(criteria)
         cursor = cursor.is_a?(cursor_type) ? cursor : new_cursor(cursor_type, cursor, cursor_options)
         raise_mismatched_sort_fields_error!(cursor, cursor_options) if different_sort_fields?(cursor, cursor_options)
-        cursor_criteria = build_cursor_criteria(criteria, cursor)
+        records = find_records(criteria, cursor)
         if block_given?
-          cursor_criteria.order_by(_id: scroll_direction(criteria)).each do |record|
-            yield record, cursor_from_record(cursor_type, record, cursor_options)
+          previous_cursor = nil
+          records.each do |record|
+            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
-          cursor_criteria
+          records
         end
       end
 
@@ -60,10 +62,21 @@ def new_cursor(cursor_type, cursor, cursor_options)
         cursor_type.new(cursor, cursor_options)
       end
 
-      def build_cursor_criteria(criteria, cursor)
+      def find_records(criteria, cursor)
         cursor_criteria = criteria.dup
         cursor_criteria.selector = { '$and' => [criteria.selector, cursor.criteria] }
-        cursor_criteria
+        if cursor.previous && criteria.options[:limit]
+          pipeline = [
+            { '$match' => cursor_criteria.selector },
+            { '$sort' => { cursor.field_name => -cursor.direction } },
+            { '$limit' => criteria.options[:limit] },
+            { '$sort' => { cursor.field_name => cursor.direction } }
+          ]
+          aggregation = cursor_criteria.view.aggregate(pipeline)
+          aggregation.map { |record| Mongoid::Factory.from_db(cursor_criteria.klass, record) }
+        else
+          cursor_criteria.order_by(_id: scroll_direction(criteria))
+        end
       end
 
       def cursor_from_record(cursor_type, record, cursor_options)

lib/mongoid/scroll/base64_encoded_cursor.rb

@@ -18,7 +18,8 @@ def initialize(value, options = {})
             field_name: parsed['field_name'],
             direction: parsed['direction'],
             include_current: parsed['include_current'],
-            tiebreak_id: parsed['tiebreak_id'] && !parsed['tiebreak_id'].empty? ? BSON::ObjectId.from_string(parsed['tiebreak_id']) : nil
+            tiebreak_id: parsed['tiebreak_id'] && !parsed['tiebreak_id'].empty? ? BSON::ObjectId.from_string(parsed['tiebreak_id']) : nil,
+            previous: parsed['previous']
           }
         else
           super nil, options
@@ -32,7 +33,8 @@ def to_s
           field_name: field_name,
           direction: direction,
           include_current: include_current,
-          tiebreak_id: tiebreak_id && tiebreak_id.to_s
+          tiebreak_id: tiebreak_id && tiebreak_id.to_s,
+          previous: previous
         }.to_json)
       end
     end

lib/mongoid/scroll/base_cursor.rb

@@ -1,7 +1,7 @@
 module Mongoid
   module Scroll
     class BaseCursor
-      attr_accessor :value, :tiebreak_id, :field_type, :field_name, :direction, :include_current
+      attr_accessor :value, :tiebreak_id, :field_type, :field_name, :direction, :include_current, :previous
 
       def initialize(value, options = {})
         @value = value
@@ -10,6 +10,7 @@ def initialize(value, options = {})
         @field_name = options[:field_name]
         @direction = options[:direction] || 1
         @include_current = options[:include_current] || false
+        @previous = options[:previous] || false
       end
 
       def criteria
@@ -86,20 +87,23 @@ def extract_field_options(options)
             field_type: field_type.to_s,
             field_name: field_name.to_s,
             direction: options[:direction] || 1,
-            include_current: options[:include_current] || false
+            include_current: options[:include_current] || false,
+            previous: options[:previous] || false
           }
         elsif options && (field = options[:field])
           {
             field_type: field.type.to_s,
             field_name: field.name.to_s,
             direction: options[:direction] || 1,
-            include_current: options[:include_current] || false
+            include_current: options[:include_current] || false,
+            previous: options[:previous] || false
           }
         end
       end
 
       def compare_direction
-        direction == 1 ? '$gt' : '$lt'
+        dir = previous ? -direction : direction
+        dir == 1 ? '$gt' : '$lt'
       end
 
       def tiebreak_compare_direction

lib/mongoid/scroll/version.rb

@@ -1,5 +1,5 @@
 module Mongoid
   module Scroll
-    VERSION = '1.0.2'.freeze
+    VERSION = '1.1.0'.freeze
   end
 end

spec/mongo/collection_view_spec.rb

@@ -106,6 +106,28 @@
                 expect(cursor.value).to eq record[field_name.to_s]
                 expect(cursor.tiebreak_id).to eq record['_id']
               end
+              it 'can scroll back with the previous cursor' do
+                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
+
+                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
+
+                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
+
+                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
         end

spec/mongoid/base64_encoded_cursor_spec.rb

@@ -3,17 +3,18 @@
 describe Mongoid::Scroll::Base64EncodedCursor do
   context 'new' do
     context 'an empty cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6bnVsbCwiZmllbGRfdHlwZSI6IlN0cmluZyIsImZpZWxkX25hbWUiOiJhX3N0cmluZyIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOm51bGx9' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6bnVsbCwiZmllbGRfdHlwZSI6IlN0cmluZyIsImZpZWxkX25hbWUiOiJhX3N0cmluZyIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOm51bGwsInByZXZpb3VzIjpmYWxzZX0=' }
       subject do
         Mongoid::Scroll::Base64EncodedCursor.new base64_string
       end
       its(:tiebreak_id) { should be_nil }
       its(:value) { should be_nil }
       its(:criteria) { should eq({}) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'a string field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6ImEgc3RyaW5nIiwiZmllbGRfdHlwZSI6IlN0cmluZyIsImZpZWxkX25hbWUiOiJhX3N0cmluZyIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2M2RmODA5NDQzNDE3YzdkMmIxMDIifQ==' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6ImEgc3RyaW5nIiwiZmllbGRfdHlwZSI6IlN0cmluZyIsImZpZWxkX25hbWUiOiJhX3N0cmluZyIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2M2RmODA5NDQzNDE3YzdkMmIxMDIiLCJwcmV2aW91cyI6ZmFsc2V9' }
       let(:a_value) { 'a string' }
       let(:tiebreak_id) { BSON::ObjectId.from_string('64063df809443417c7d2b102') }
       let(:criteria) do
@@ -32,10 +33,11 @@
       its(:value) { should eq a_value }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'an id field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6IjY0MDY0NTg0MDk0NDM0MjgxZmE3MWFiMiIsImZpZWxkX3R5cGUiOiJCU09OOjpPYmplY3RJZCIsImZpZWxkX25hbWUiOiJpZCIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2NDU4NDA5NDQzNDI4MWZhNzFhYjIifQ==' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6IjY0MDY0NTg0MDk0NDM0MjgxZmE3MWFiMiIsImZpZWxkX3R5cGUiOiJCU09OOjpPYmplY3RJZCIsImZpZWxkX25hbWUiOiJpZCIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2NDU4NDA5NDQzNDI4MWZhNzFhYjIiLCJwcmV2aW91cyI6ZmFsc2V9' }
       let(:a_value) { BSON::ObjectId('64064584094434281fa71ab2') }
       let(:tiebreak_id) { a_value }
       let(:criteria) do
@@ -52,10 +54,11 @@
       its(:value) { should eq a_value }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'an integer field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6MTAsImZpZWxkX3R5cGUiOiJJbnRlZ2VyIiwiZmllbGRfbmFtZSI6ImFfaW50ZWdlciIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2M2RmODA5NDQzNDE3YzdkMmIxMDgifQ==' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6MTAsImZpZWxkX3R5cGUiOiJJbnRlZ2VyIiwiZmllbGRfbmFtZSI6ImFfaW50ZWdlciIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2M2RmODA5NDQzNDE3YzdkMmIxMDgiLCJwcmV2aW91cyI6ZmFsc2V9' }
       let(:a_value) { 10 }
       let(:tiebreak_id) { BSON::ObjectId('64063df809443417c7d2b108') }
       let(:criteria) do
@@ -74,10 +77,11 @@
       its(:value) { should eq a_value }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'a date/time field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzU5MDEyMy4wLCJmaWVsZF90eXBlIjoiRGF0ZVRpbWUiLCJmaWVsZF9uYW1lIjoiYV9kYXRldGltZSIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2NDNhNzA5NDQzNDIzOWYyZGJmODYifQ==' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzU5MDEyMy4wLCJmaWVsZF90eXBlIjoiRGF0ZVRpbWUiLCJmaWVsZF9uYW1lIjoiYV9kYXRldGltZSIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOiI2NDA2NDNhNzA5NDQzNDIzOWYyZGJmODYiLCJwcmV2aW91cyI6ZmFsc2V9' }
       let(:a_value) { DateTime.new(2013, 12, 21, 1, 42, 3, 'UTC') }
       let(:tiebreak_id) { BSON::ObjectId('640643a7094434239f2dbf86') }
       let(:criteria) do
@@ -94,10 +98,11 @@
       its(:value) { should eq a_value }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'a date field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzU4NDAwMCwiZmllbGRfdHlwZSI6IkRhdGUiLCJmaWVsZF9uYW1lIjoiYV9kYXRlIiwiZGlyZWN0aW9uIjoxLCJpbmNsdWRlX2N1cnJlbnQiOmZhbHNlLCJ0aWVicmVha19pZCI6IjY0MDY0MmM5MDk0NDM0MjEyYzRkNDQyMCJ9' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzU4NDAwMCwiZmllbGRfdHlwZSI6IkRhdGUiLCJmaWVsZF9uYW1lIjoiYV9kYXRlIiwiZGlyZWN0aW9uIjoxLCJpbmNsdWRlX2N1cnJlbnQiOmZhbHNlLCJ0aWVicmVha19pZCI6IjY0MDY0MmM5MDk0NDM0MjEyYzRkNDQyMCIsInByZXZpb3VzIjpmYWxzZX0=' }
       let(:tiebreak_id) { BSON::ObjectId('640642c9094434212c4d4420') }
       let(:a_value) { Date.new(2013, 12, 21) }
       let(:criteria) do
@@ -114,10 +119,11 @@
       its(:value) { should eq a_value }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'a time field cursor' do
-      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzYwNTcyMy4wLCJmaWVsZF90eXBlIjoiVGltZSIsImZpZWxkX25hbWUiOiJhX3RpbWUiLCJkaXJlY3Rpb24iOjEsImluY2x1ZGVfY3VycmVudCI6ZmFsc2UsInRpZWJyZWFrX2lkIjoiNjQwNjNkNGEwOTQ0MzQxNjZiZDA1M2VkIn0=' }
+      let(:base64_string) { 'eyJ2YWx1ZSI6MTM4NzYwNTcyMy4wLCJmaWVsZF90eXBlIjoiVGltZSIsImZpZWxkX25hbWUiOiJhX3RpbWUiLCJkaXJlY3Rpb24iOjEsImluY2x1ZGVfY3VycmVudCI6ZmFsc2UsInRpZWJyZWFrX2lkIjoiNjQwNjNkNGEwOTQ0MzQxNjZiZDA1M2VkIiwicHJldmlvdXMiOmZhbHNlfQ==' }
       let(:item_id) { BSON::ObjectId('640636f209443407333b46d4') }
       let(:a_value) { Time.new(2013, 12, 21, 6, 2, 3, '+00:00').utc }
       let(:tiebreak_id) { BSON::ObjectId('64063d4a094434166bd053ed') }
@@ -136,6 +142,7 @@
       its(:tiebreak_id) { tiebreak_id }
       its(:tiebreak_id) { should eq tiebreak_id }
       its(:criteria) { should eq(criteria) }
+      its(:previous) { should be_falsy }
       its(:to_s) { should eq(base64_string) }
     end
     context 'an invalid field cursor' do
@@ -229,5 +236,22 @@
         end.to raise_error Mongoid::Scroll::Errors::UnsupportedFieldTypeError, /The type of the field 'a_array' is not supported: Array./
       end
     end
+
+    it 'encode and decode previous option' do
+      feed_item = Feed::Item.create!
+      cursor = Mongoid::Scroll::Base64EncodedCursor.from_record feed_item, field_name: 'id', field_type: BSON::ObjectId, previous: true
+      expect(Mongoid::Scroll::Base64EncodedCursor.new(cursor.to_s).previous).to be_truthy
+    end
+    context 'a cursor with previous set to true' do
+      let(:field_type) { BSON::ObjectId }
+      let(:field_name) { 'id' }
+      let(:feed_item) { Feed::Item.create! }
+      subject do
+        Mongoid::Scroll::Base64EncodedCursor.from_record feed_item, field_name: field_name, field_type: field_type, previous: true
+      end
+      its(:value) { should eq feed_item._id }
+      its(:field_type) { should eq field_type.to_s }
+      its(:previous) { should be_truthy }
+    end
   end
 end

spec/mongoid/criteria_spec.rb

@@ -160,6 +160,28 @@
                 break
               end
             end
+            it 'can scroll back with the previous cursor' do
+              cursor = nil
+              first_previous_cursor = nil
+              second_previous_cursor = nil
+
+              Feed::Item.asc(field_name).limit(2).scroll(cursor_type) do |_, next_cursor|
+                cursor = next_cursor
+              end
+
+              Feed::Item.asc(field_name).limit(2).scroll(cursor) do |_, next_cursor, previous_cursor|
+                cursor = next_cursor
+                first_previous_cursor = previous_cursor
+              end
+
+              Feed::Item.asc(field_name).limit(2).scroll(cursor) do |_, _, previous_cursor|
+                second_previous_cursor = previous_cursor
+              end
+
+              records = Feed::Item.asc(field_name)
+              expect(Feed::Item.asc(field_name).limit(2).scroll(first_previous_cursor)).to eq(records.limit(2))
+              expect(Feed::Item.asc(field_name).limit(2).scroll(second_previous_cursor)).to eq(records.skip(2).limit(2))
+            end
           end
         end
       end