ValueError: by adding custom aggregate function TRIM

[zegrep@…]

My intension is joining tables with two different formated columns,
by using django's double underscore magic to resolve the querysets.

The values in table B are preceded by leading zeros.

table A --> id = "10"

table B --> id = "000010"

select distinct b.id from a, b where where a.id = TRIM(leading '0' FROM b.id);

The resulting code should look like this

    qs_a=A.objects.all().values('id')
    qs_b=B.objects.annotate(id_trim=Trim('id', position='leading', char='0')).filter(id_trim__in=qs_a)

I use the following code to implement the extra functionality.

try:
    import psycopg2
except ImportError, e:
    import psycopg
else:
    psycopg = psycopg2._psycopg

class Trim(django.db.models.Aggregate):
    name = 'Trim_pg'
django.db.models.Trim=Trim

class Trim_pg(django.db.models.sql.aggregates.Aggregate):
    '''
        position = [leading, trailing, both]
        char = <character to remove>
    '''

    sql_function = 'TRIM'
    sql_template = '''%(function)s(%(position)s %(char)s FROM %(field)s)'''

    def __init__(self, col, distinct=False, **extra):
        assert extra.has_key('position'), u'no position'
        assert extra['position'] in ('leading', 'trailing', 'both'), 'position no in [leading, trailing, both]'
        assert extra.has_key('char'), u'no char'
        assert len(extra['char']) == 1, 'only one character'
        extra['char']=str(psycopg2._psycopg.QuotedString(extra['char'])) #Quoting
        super(Trim_pg, self).__init__(col, distinct=distinct, **extra)
django.db.models.sql.aggregates.Trim_pg=Trim_pg

The problem is, that "convert_values" makes for a "CharField"
a cast to float. My solution is to return the value for CharFields
without the cast.

Index: db/backends/__init__.py
===================================================================
--- db/backends/__init__.py (Revision 12595)
+++ db/backends/__init__.py (Arbeitskopie)
@@ -438,6 +438,8 @@
             return int(value)
         elif internal_type in ('DateField', 'DateTimeField', 'TimeField'):
             return value
+        elif internal_type in  ('CharField'):
+            return value
         # No field, or the field isn't known to be a decimal or integer
         # Default to a float
         return float(value)