From fbff08406dffdc954bc70268b8d8031254ba7ccc Mon Sep 17 00:00:00 2001
From: fatkodima <fatkodima123@gmail.com>
Date: Sat, 18 Jan 2025 16:56:52 +0200
Subject: [PATCH] Add ability to paginate backward from the end of the
 collection

---
 CHANGELOG.md                                  |  6 +++++
 lib/activerecord_cursor_paginate/paginator.rb | 26 ++++++++++++-------
 test/paginator_test.rb                        | 10 +++++++
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7ae3998..009475b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 ## master (unreleased)
 
+- Add ability to paginate backward from the end of the collection
+
+    ```ruby
+    paginator = users.cursor_paginate(forward_pagination: false)
+    ```
+
 ## 0.3.0 (2025-01-06)
 
 - Allow paginating over nullable columns
diff --git a/lib/activerecord_cursor_paginate/paginator.rb b/lib/activerecord_cursor_paginate/paginator.rb
index 8feabe0..8cba5f4 100644
--- a/lib/activerecord_cursor_paginate/paginator.rb
+++ b/lib/activerecord_cursor_paginate/paginator.rb
@@ -20,6 +20,7 @@ module ActiveRecordCursorPaginate
   #
   class Paginator
     attr_reader :relation, :before, :after, :limit, :order, :append_primary_key
+    attr_accessor :forward_pagination
 
     # Create a new instance of the `ActiveRecordCursorPaginate::Paginator`
     #
@@ -45,10 +46,14 @@ class Paginator
     #   It is not recommended to use this feature, because the complexity of produced SQL
     #   queries can have a very negative impact on the database performance. It is better
     #   to paginate using only non-nullable columns.
+    # @param forward_pagination [Boolean] Whether this is a forward or backward pagination.
+    #   Optional, defaults to `true` if `:before` is not provided, `false` otherwise.
+    #   Useful when paginating backward from the end of the collection.
     #
     # @raise [ArgumentError] If any parameter is not valid
     #
-    def initialize(relation, before: nil, after: nil, limit: nil, order: nil, append_primary_key: true, nullable_columns: nil)
+    def initialize(relation, before: nil, after: nil, limit: nil, order: nil, append_primary_key: true,
+                   nullable_columns: nil, forward_pagination: before.nil?)
       unless relation.is_a?(ActiveRecord::Relation)
         raise ArgumentError, "relation is not an ActiveRecord::Relation"
       end
@@ -58,7 +63,7 @@ def initialize(relation, before: nil, after: nil, limit: nil, order: nil, append
       @append_primary_key = append_primary_key
 
       @cursor = @current_cursor = nil
-      @is_forward_pagination = true
+      @forward_pagination = forward_pagination
       @before = @after = nil
       @page_size = nil
       @limit = nil
@@ -80,17 +85,18 @@ def before=(value)
 
       @cursor = value || after
       @current_cursor = @cursor
-      @is_forward_pagination = value.blank?
+      @forward_pagination = false if value
       @before = value
     end
 
     def after=(value)
-      if before.present? && value.present?
+      if value.present? && before.present?
         raise ArgumentError, "Only one of :before and :after can be provided"
       end
 
-      @cursor = before || value
+      @cursor = value || before
       @current_cursor = @cursor
+      @forward_pagination = true if value
       @after = value
     end
 
@@ -136,9 +142,9 @@ def fetch
       has_additional = records_plus_one.size > @page_size
 
       records = records_plus_one.take(@page_size)
-      records.reverse! unless @is_forward_pagination
+      records.reverse! unless @forward_pagination
 
-      if @is_forward_pagination
+      if @forward_pagination
         has_next_page = has_additional
         has_previous_page = @current_cursor.present?
       else
@@ -301,7 +307,7 @@ def arel_column(column)
       end
 
       def pagination_direction(direction)
-        if @is_forward_pagination
+        if @forward_pagination
           direction
         else
           direction == :asc ? :desc : :asc
@@ -309,7 +315,7 @@ def pagination_direction(direction)
       end
 
       def pagination_operator(direction)
-        if @is_forward_pagination
+        if @forward_pagination
           direction == :asc ? :gt : :lt
         else
           direction == :asc ? :lt : :gt
@@ -318,7 +324,7 @@ def pagination_operator(direction)
 
       def advance_by_page(page)
         @current_cursor =
-          if @is_forward_pagination
+          if @forward_pagination
             page.next_cursor
           else
             page.previous_cursor
diff --git a/test/paginator_test.rb b/test/paginator_test.rb
index b56bda7..dc5c91c 100644
--- a/test/paginator_test.rb
+++ b/test/paginator_test.rb
@@ -80,6 +80,16 @@ def test_paginating_by_multiple_nullable_cursor_columns_in_desc_order
     assert_equal Project.order(name: :desc, organization_id: :asc, id: :asc).to_a, records
   end
 
+  def test_paginating_backward_from_the_end
+    p = User.cursor_paginate(limit: 3, forward_pagination: false)
+
+    ids = p.pages.map do |page|
+      page.records.pluck(:id)
+    end
+
+    assert_equal [[8, 9, 10], [5, 6, 7], [2, 3, 4], [1]], ids
+  end
+
   def test_uses_default_limit
     ActiveRecordCursorPaginate.config.stub(:default_page_size, 4) do
       p = User.cursor_paginate