custom django aggregate

django custom aggregate

        class MyMax(Aggregate):
            sql_function = 'CHAR_LENGTH'
            """A base class to make it easy for end users to define their own
            custom SQL aggregates.
        
            The subclass should define the following two class properties:
             * sql_function - the name of the SQL function to invoke
        
            Optionally, you can define
              * sql_template - a format string that is used to compose the
                SQL that will be sent to the database. The template will be
                provided with the following substitution variables:
                  - ``function``, the sql fuction that will be invoked
                  - ``field``, the resolved name of the column to be
                    operated on.
                The template will also be provided with any keyword argument
                provided to the aggregate when it was defined.
                The default template is '%(function)s(%(field)s)'
              * is_ordinal - a boolean, True if the result of the aggregate
                will always be a count, regardless of the field on which the
                aggregate is applied. False by default.
              * is_computed - a boolean, True if the result of the aggregate
                will always be a float, regardless of the field on which the
                aggregate is applied. False by default.
            """
            def __init__(self, lookup, **extra):
                self.lookup = lookup
                self.extra = extra
        
            def _default_alias(self):
                return '%s__%s' % (self.lookup, self.__class__.__name__.lower())
            default_alias = property(_default_alias)
        
            def add_to_query(self, query, alias, col, source, is_summary):
                super(MyMax, self).__init__(col, source, is_summary, **self.extra)
                query.aggregate_select[alias] = self