Implement the ability to preserve ordering of cursor

Author: RTS340

.gitignore

@@ -2,3 +2,4 @@ build/
 dist/
 *.egg-info/
 *.pyc
+.venv/

cursor_pagination.py

@@ -5,6 +5,113 @@
 from django.utils.translation import gettext_lazy as _
 
 
+class CursorStrategy:
+    """Base interface for cursor pagination strategies."""
+
+    def get_ordering(self, ordering, from_last=False):
+        """Transform ordering fields for database queries according to the strategy."""
+        raise NotImplementedError
+
+    def build_cursor_filter(
+        self, ordering, cursor_values, reverse=False, from_last=False
+    ):
+        """Build the cursor filter using the strategy's approach."""
+        raise NotImplementedError
+
+
+class DefaultCursorStrategy(CursorStrategy):
+    """Default strategy maintaining current NULLS LAST behavior."""
+
+    def get_ordering(self, ordering, from_last=False):
+        """
+        Transform ordering fields with explicit NULL handling for consistent behavior.
+
+        This clarifies that NULL values come at the end in the sort.
+        When "from_last" is specified, NULL values come first since we return
+        the results in reversed order.
+        """
+        nulls_ordering = []
+        for key in ordering:
+            is_reversed = key.startswith('-')
+            column = key.lstrip('-')
+            if is_reversed:
+                if from_last:
+                    nulls_ordering.append(F(column).desc(nulls_first=True))
+                else:
+                    nulls_ordering.append(F(column).desc(nulls_last=True))
+            else:
+                if from_last:
+                    nulls_ordering.append(F(column).asc(nulls_first=True))
+                else:
+                    nulls_ordering.append(F(column).asc(nulls_last=True))
+
+        return nulls_ordering
+
+    def build_cursor_filter(
+        self, ordering, cursor_values, reverse=False, from_last=False
+    ):
+        """
+        Build the cursor filter using the current OR logic and NULL handling.
+        This is the existing implementation from the apply_cursor method.
+        """
+        if not ordering or not cursor_values:
+            return Q()
+
+        if len(ordering) != len(cursor_values):
+            raise ValueError("Ordering and cursor values must match length")
+
+        # Convert cursor values for comparison
+        position_values = [
+            Value(pos, output_field=TextField()) if pos is not None else None
+            for pos in cursor_values
+        ]
+
+        # Build Q object with OR logic and NULL handling (current implementation)
+        filtering = Q()
+        q_equality = {}
+
+        for ordering_field, value in zip(ordering, position_values):
+            is_reversed = ordering_field.startswith('-')
+            o = ordering_field.lstrip('-')
+            if value is None:  # cursor value for the key was NULL
+                key = "{}__isnull".format(o)
+                if (
+                    from_last is True
+                ):  # if from_last & cursor value is NULL, we need to get non Null for the key
+                    q = {key: False}
+                    q.update(q_equality)
+                    filtering |= Q(**q)
+
+                q_equality.update({key: True})
+            else:  # cursor value for the key was non NULL
+                if reverse != is_reversed:
+                    comparison_key = "{}__lt".format(o)
+                else:
+                    comparison_key = "{}__gt".format(o)
+
+                q = Q(**{comparison_key: value})
+                if not from_last:  # if not from_last, NULL values are still candidates
+                    q |= Q(**{"{}__isnull".format(o): True})
+                filtering |= (q) & Q(**q_equality)
+
+                equality_key = "{}__exact".format(o)
+                q_equality.update({equality_key: value})
+
+        return filtering
+
+
+class PreserveOrderingStrategy(DefaultCursorStrategy):
+    """
+    Cursor strategy that preserves the ordering of the fields.
+    """
+
+    def get_ordering(self, ordering, from_last=False):
+        """
+        Return simple ordering fields.
+        """
+        return ordering
+
+
 class InvalidCursor(Exception):
     pass
 
@@ -14,6 +121,7 @@ def reverse_ordering(ordering_tuple):
     Given an order_by tuple such as `('-created', 'uuid')` reverse the
     ordering and return a new tuple, eg. `('created', '-uuid')`.
     """
+
     def invert(x):
         return x[1:] if (x.startswith('-')) else '-' + x
 
@@ -34,41 +142,33 @@ def __getitem__(self, key):
         return self.items.__getitem__(key)
 
     def __repr__(self):
-        return '<Page: [%s%s]>' % (', '.join(repr(i) for i in self.items[:21]), ' (remaining truncated)' if len(self.items) > 21 else '')
+        return '<Page: [%s%s]>' % (
+            ', '.join(repr(i) for i in self.items[:21]),
+            ' (remaining truncated)' if len(self.items) > 21 else '',
+        )
 
 
 class CursorPaginator(object):
     delimiter = '|'
     none_string = '::None'
     invalid_cursor_message = _('Invalid cursor')
 
-    def __init__(self, queryset, ordering):
-        self.queryset = queryset.order_by(*self._nulls_ordering(ordering))
+    def __init__(self, queryset, ordering, strategy=None):
         self.ordering = ordering
+        self.strategy = strategy or DefaultCursorStrategy()
+        self.queryset = queryset.order_by(*self._get_ordering(ordering))
 
-    def _nulls_ordering(self, ordering, from_last=False):
-        """
-        This clarifies that NULL value comes at the end in the sort.
-        When "from_last" is specified, NULL value comes first since we return the results in reversed order.
-        """
-        nulls_ordering = []
-        for key in ordering:
-            is_reversed = key.startswith('-')
-            column = key.lstrip('-')
-            if is_reversed:
-                if from_last:
-                    nulls_ordering.append(F(column).desc(nulls_first=True))
-                else:
-                    nulls_ordering.append(F(column).desc(nulls_last=True))
-            else:
-                if from_last:
-                    nulls_ordering.append(F(column).asc(nulls_first=True))
-                else:
-                    nulls_ordering.append(F(column).asc(nulls_last=True))
+    def _get_ordering(self, ordering):
+        """Get database ordering using the current strategy."""
+        return self.strategy.get_ordering(ordering)
 
-        return nulls_ordering
+    def _nulls_ordering(self, ordering, from_last=False):
+        """Deprecated: Use strategy.get_ordering instead."""
+        return self.strategy.get_ordering(ordering, from_last)
 
-    def _apply_paginator_arguments(self, qs, first=None, last=None, after=None, before=None):
+    def _apply_paginator_arguments(
+        self, qs, first=None, last=None, after=None, before=None
+    ):
         """
         Apply first/after, last/before filtering to the queryset
         """
@@ -81,9 +181,11 @@ def _apply_paginator_arguments(self, qs, first=None, last=None, after=None, befo
         if before is not None:
             qs = self.apply_cursor(before, qs, from_last=from_last, reverse=True)
         if first is not None:
-            qs = qs[:first + 1]
+            qs = qs[: first + 1]
         if last is not None:
-            qs = qs.order_by(*self._nulls_ordering(reverse_ordering(self.ordering), from_last=True))[:last + 1]
+            qs = qs.order_by(
+                *self._nulls_ordering(reverse_ordering(self.ordering), from_last=True)
+            )[: last + 1]
 
         return qs
 
@@ -128,90 +230,28 @@ async def apage(self, first=None, last=None, after=None, before=None):
         return self._get_cursor_page(items, has_additional, first, last, after, before)
 
     def apply_cursor(self, cursor, queryset, from_last, reverse=False):
+        """Apply cursor using the current strategy."""
         position = self.decode_cursor(cursor)
-
-        # this was previously implemented as tuple comparison done on postgres side
-        # Assume comparing 3-tuples a and b,
-        # the comparison a < b is equivalent to:
-        # (a.0 < b.0) || (a.0 == b.0 && (a.1 < b.1)) || (a.0 == b.0 && a.1 == b.1 && (a.2 < b.2))
-        # The expression above does not depend on short-circuit evalution support,
-        # which is usually unavailable on backend RDB
-
-        # In order to reflect that in DB query,
-        # we need to generate a corresponding WHERE-clause.
-
-        # Suppose we have ordering ("field1", "-field2", "field3")
-        # (note negation 2nd item),
-        # and corresponding cursor values are ("value1", "value2", "value3"),
-        # `reverse` is False.
-        # In order to apply cursor, we need to generate a following WHERE-clause:
-
-        # WHERE ((field1 < value1 OR field1 IS NULL) OR
-        #     (field1 = value1 AND (field2 > value2 OR field2 IS NULL)) OR
-        #     (field1 = value1 AND field2 = value2 AND (field3 < value3 IS NULL)).
-        #
-        # Keep in mind, NULL is considered the last part of each field's order. 
-        # We will use `__lt` lookup for `<`,
-        # `__gt` for `>` and `__exact` for `=`.
-        # (Using case-sensitive comparison as long as
-        # cursor values come from the DB against which it is going to be compared).
-        # The corresponding django ORM construct would look like:
-        # filter(
-        #     Q(field1__lt=Value(value1) OR field1__isnull=True) |
-        #     Q(field1__exact=Value(value1), (Q(field2__gt=Value(value2) | Q(field2__isnull=True)) |
-        #     Q(field1__exact=Value(value1), field2__exact=Value(value2), (Q(field3__lt=Value(value3) | Q(field3__isnull=True)))
-        # )
-
-        # In order to remember which keys we need to compare for equality on the next iteration,
-        # we need an accumulator in which we store all the previous keys.
-        # When we are generating a Q object for j-th position/ordering pair,
-        # our q_equality would contain equality lookups
-        # for previous pairs of 0-th to (j-1)-th pairs.
-        # That would allow us to generate a Q object like the following:
-        # Q(f1__exact=Value(v1), f2__exact=Value(v2), ..., fj_1__exact=Value(vj_1), fj__lt=Value(vj)),
-        # where the last item would depend on both "reverse" option and ordering key sign.
-
-        filtering = Q()
-        q_equality = {}
-
-        position_values = [Value(pos, output_field=TextField()) if pos is not None else None for pos in position]
-
-        for ordering, value in zip(self.ordering, position_values):
-            is_reversed = ordering.startswith('-')
-            o = ordering.lstrip('-')
-            if value is None:  # cursor value for the key was NULL
-                key = "{}__isnull".format(o)
-                if from_last is True:  # if from_last & cursor value is NULL, we need to get non Null for the key
-                    q = {key : False}
-                    q.update(q_equality)
-                    filtering |= Q(**q)
-
-                q_equality.update({key: True})
-            else:  # cursor value for the key was non NULL
-                if reverse != is_reversed:
-                    comparison_key = "{}__lt".format(o)
-                else:
-                    comparison_key = "{}__gt".format(o)
-
-                q = Q(**{comparison_key: value})
-                if not from_last:  # if not from_last, NULL values are still candidates
-                     q |= Q(**{"{}__isnull".format(o): True})
-                filtering |= (q) & Q(**q_equality)
-
-                equality_key = "{}__exact".format(o)
-                q_equality.update({equality_key: value})
-
-        return queryset.filter(filtering)
+        return queryset.filter(
+            self.strategy.build_cursor_filter(
+                self.ordering, position, reverse, from_last
+            )
+        )
 
     def decode_cursor(self, cursor):
         try:
             orderings = b64decode(cursor.encode('ascii')).decode('utf8')
-            return [ordering if ordering != self.none_string else None for ordering in orderings.split(self.delimiter)]
+            return [
+                ordering if ordering != self.none_string else None
+                for ordering in orderings.split(self.delimiter)
+            ]
         except (TypeError, ValueError):
             raise InvalidCursor(self.invalid_cursor_message)
 
     def encode_cursor(self, position):
-        encoded = b64encode(self.delimiter.join(position).encode('utf8')).decode('ascii')
+        encoded = b64encode(self.delimiter.join(position).encode('utf8')).decode(
+            'ascii'
+        )
         return encoded
 
     def position_from_instance(self, instance):
@@ -231,3 +271,12 @@ def position_from_instance(self, instance):
     def cursor(self, instance):
         return self.encode_cursor(self.position_from_instance(instance))
 
+    @classmethod
+    def for_preserve_ordering(cls, queryset, ordering):
+        """Create cursor paginator using PreserveOrderingStrategy."""
+        return cls(queryset, ordering, strategy=PreserveOrderingStrategy())
+
+    @classmethod
+    def with_strategy(cls, queryset, ordering, strategy):
+        """Create a cursor paginator with a custom strategy."""
+        return cls(queryset, ordering, strategy=strategy)

runtests.py

@@ -6,7 +6,6 @@
 from django.conf import settings
 from django.test.utils import get_runner
 
-
 if __name__ == '__main__':
     os.environ['DJANGO_SETTINGS_MODULE'] = 'tests.settings'
     django.setup()

setup.py

@@ -1,6 +1,5 @@
 from setuptools import setup
 
-
 with open("README.md", "r") as fh:
     long_description = fh.read()