GREATEST() of aggregates, in the Django ORM.

greatest.py

class Greatest(object):
    """A pseudo- (or meta-) aggregate; returns highest of sub-aggregates.

    This has to be inserted directly into the Query internals; it can't go
    through the usual ``qs.annotate(...)`` or ``qs.aggregate(...)`` path
    (because that leads to ``Query.add_aggregate(...)`` which makes assumptions
    we can't meet).

    To use, instantiate ``Greatest`` with one or more normal aggregates, and
    call ``greatest_instance.add_to_qs(queryset, 'name')``

    All aggregates passed to ``Greatest`` must be based on the same underlying
    database field type (i.e. char, int, datetime, whatever), or else things
    will probably explode.

    """
    is_ordinal = False
    is_computed = False

    def __init__(self, *aggregates):
        self.aggregates = aggregates
        self.sql_aggregates = []

    def add_to_qs(self, queryset, alias):
        """Add to the nominated queryset."""
        for aggregate in self.aggregates:
            queryset.query.add_aggregate(
                aggregate, queryset.model, 'tmpalias', False)
            self.sql_aggregates.append(
                queryset.query.aggregates.pop('tmpalias'))
        queryset._setup_aggregate_query([alias])
        queryset.query.aggregates[alias] = self

    @cached_property
    def field(self):
        return self.sql_aggregates[0].field

    def as_sql(self, qn, connection):
        sql_bits = []
        params = []
        for a in self.sql_aggregates:
            sql, p = a.as_sql(qn, connection)
            sql_bits.append(sql)
            params.extend(p)
        return "GREATEST(%s)" % (', '.join(sql_bits)), params