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from django.db.models.expressions import F, OrderBy
class OrderableAggMixin:
def __init__(self, expression, ordering=(), **extra):
if not isinstance(ordering, (list, tuple)):
ordering = [ordering]
ordering = ordering or []
# Transform minus sign prefixed strings into an OrderBy() expression.
ordering = (
(OrderBy(F(o[1:]), descending=True) if isinstance(o, str) and o[0] == '-' else o)
for o in ordering
)
super().__init__(expression, **extra)
self.ordering = self._parse_expressions(*ordering)
def resolve_expression(self, *args, **kwargs):
self.ordering = [expr.resolve_expression(*args, **kwargs) for expr in self.ordering]
return super().resolve_expression(*args, **kwargs)
def as_sql(self, compiler, connection):
if self.ordering:
self.extra['ordering'] = 'ORDER BY ' + ', '.join((
ordering_element.as_sql(compiler, connection)[0]
for ordering_element in self.ordering
))
else:
self.extra['ordering'] = ''
return super().as_sql(compiler, connection)
def get_source_expressions(self):
return self.source_expressions + self.ordering
def get_source_fields(self):
# Filter out fields contributed by the ordering expressions as
# these should not be used to determine which the return type of the
# expression.
return [
e._output_field_or_none
for e in self.get_source_expressions()[:self._get_ordering_expressions_index()]
]
def _get_ordering_expressions_index(self):
"""Return the index at which the ordering expressions start."""
source_expressions = self.get_source_expressions()
return len(source_expressions) - len(self.ordering)