Raise NoSuchFieldError for missing sort and cursor fields

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### 2.0.1 (Next)
 
+* [#48](https://github.com/mongoid/mongoid-scroll/issues/48): Raise `NoSuchFieldError` when a sort or cursor refers to a field no longer defined on the model - [@dblock](https://github.com/dblock).
 * [#51](https://github.com/mongoid/mongoid-scroll/pull/51): Report SimpleCov coverage to Coveralls once per build without duplicate gem uploads and refresh CI and coverage badges - [@dblock](https://github.com/dblock).
 * [#50](https://github.com/mongoid/mongoid-scroll/pull/50): Modernize CI through Ruby 4.0, Mongoid 9 and MongoDB 8; fix Mongoid 9 dependency selection, database cleaner compatibility and invalid Danger workflow references - [@dblock](https://github.com/dblock).
 * [#49](https://github.com/mongoid/mongoid-scroll/pull/49): Migrate Danger to danger-pr-comment workflow - [@dblock](https://github.com/dblock).

README.md

@@ -74,6 +74,8 @@ end
 
 Resume iterating using the saved cursor and save the cursor to go backwards.
 
+If the sort field or a saved cursor's field is no longer defined on the model, `scroll` raises `Mongoid::Scroll::Errors::NoSuchFieldError`. A defined field with a `nil` value is not considered missing.
+
 ```ruby
 Feed::Item.desc(:position).limit(5).scroll(saved_iterator.next_cursor) do |record, iterator|
   # each record, one-by-one

lib/mongoid/criteria/scrollable.rb

@@ -92,7 +92,7 @@ def cursor_from_record(cursor_type, record, cursor_options)
 
       def scroll_field_type(criteria)
         scroll_field = scroll_field(criteria)
-        field = criteria.klass.fields[scroll_field.to_s]
+        field = scroll_model_field(scroll_field)
         field.foreign_key? && field.object_id_field? ? BSON::ObjectId : field.type
       end
     end

lib/mongoid/criteria/scrollable/fields.rb

@@ -5,12 +5,18 @@ module Scrollable
       module Fields
         private
 
+        def scroll_model_field(field_name)
+          klass.fields[field_name.to_s] ||
+            raise(Mongoid::Scroll::Errors::NoSuchFieldError.new(field: field_name))
+        end
+
         def raise_mismatched_sort_fields_error!(cursor, criteria_cursor_options)
           diff = cursor.sort_options.reject { |k, v| criteria_cursor_options[k] == v }
           raise Mongoid::Scroll::Errors::MismatchedSortFieldsError.new(diff: diff)
         end
 
         def different_sort_fields?(cursor, criteria_cursor_options)
+          scroll_model_field(cursor.field_name)
           criteria_cursor_options[:field_type] = criteria_cursor_options[:field_type].to_s
           criteria_cursor_options[:field_name] = criteria_cursor_options[:field_name].to_s
           criteria_cursor_options != cursor.sort_options

spec/mongoid/criteria_spec.rb

@@ -42,6 +42,37 @@
         end
       end
 
+      context 'with a missing field' do
+        it 'rejects a sort field that is not defined on the model' do
+          expect do
+            Feed::Item.asc(:removed_field).scroll(cursor_type)
+          end.to raise_error Mongoid::Scroll::Errors::NoSuchFieldError, /does not exist: removed_field/
+        end
+
+        it 'rejects a cursor whose field is no longer defined on the model' do
+          cursor = cursor_type.new(nil, field_name: 'removed_field', field_type: String)
+          expect do
+            Feed::Item.asc(:name).scroll(cursor)
+          end.to raise_error Mongoid::Scroll::Errors::NoSuchFieldError, /does not exist: removed_field/
+        end
+
+        it 'does not confuse a nil field value with a missing field' do
+          record = Feed::Item.create!
+          cursor = cursor_type.from_record(record, field: record.fields['name'])
+          expect { Feed::Item.asc(:name).scroll(cursor) }.not_to raise_error
+        end
+
+        if cursor_type == Mongoid::Scroll::Base64EncodedCursor
+          it 'rejects a serialized cursor whose field is no longer defined on the model' do
+            cursor = cursor_type.new(nil, field_name: 'removed_field', field_type: String)
+            saved_cursor = cursor_type.new(cursor.to_s)
+            expect do
+              Feed::Item.asc(:name).scroll(saved_cursor)
+            end.to raise_error Mongoid::Scroll::Errors::NoSuchFieldError, /does not exist: removed_field/
+          end
+        end
+      end
+
       context 'with data' do
         before do
           10.times do |i|