cursor_pagination.py

from base64 import b64decode, b64encode
from collections.abc import Sequence

from django.db.models import F, Q, TextField, Value
from django.utils.translation import gettext_lazy as _


class InvalidCursor(Exception):
    pass


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

    return tuple([invert(item) for item in ordering_tuple])


class CursorPage(Sequence):
    def __init__(self, items, paginator, has_next=False, has_previous=False):
        self.items = items
        self.paginator = paginator
        self.has_next = has_next
        self.has_previous = has_previous

    def __len__(self):
        return len(self.items)

    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 '')


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))
        self.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))

        return nulls_ordering



    def page(self, first=None, last=None, after=None, before=None):
        qs = self.queryset
        page_size = first or last
        if page_size is None:
            return CursorPage(qs, self)

        from_last = last is not None
        if from_last and first is not None:
            raise ValueError('Cannot process first and last') 

        if after is not None:
            qs = self.apply_cursor(after, qs, from_last=from_last)
        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]
        if last is not None:
            qs = qs.order_by(*self._nulls_ordering(reverse_ordering(self.ordering), from_last=True))[:last + 1]

        qs = list(qs)
        items = qs[:page_size]
        if last is not None:
            items.reverse()
        has_additional = len(qs) > len(items)
        additional_kwargs = {}
        if first is not None:
            additional_kwargs['has_next'] = has_additional
            additional_kwargs['has_previous'] = bool(after)
        elif last is not None:
            additional_kwargs['has_previous'] = has_additional
            additional_kwargs['has_next'] = bool(before)
        return CursorPage(items, self, **additional_kwargs)

    def apply_cursor(self, cursor, queryset, from_last, reverse=False):
        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)

    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)]
        except (TypeError, ValueError):
            raise InvalidCursor(self.invalid_cursor_message)

    def encode_cursor(self, position):
        encoded = b64encode(self.delimiter.join(position).encode('utf8')).decode('ascii')
        return encoded

    def position_from_instance(self, instance):
        position = []
        for order in self.ordering:
            parts = order.lstrip('-').split('__')
            attr = instance
            while parts:
                attr = getattr(attr, parts[0])
                parts.pop(0)
            if attr is None:
                position.append(self.none_string)
            else:
                position.append(str(attr))
        return position

    def cursor(self, instance):
        return self.encode_cursor(self.position_from_instance(instance))