Hey everyone,
I did a little more digging and came up with a solution by extending
the default manager to add a new function 'get_orphans'. When passed a
M2M non-symmetrical field, it will do a query and return a query set
containing all items which have no parent - ie. any item that does not
appear as an id in the 'to_*_id' field in the M2M relation db. Below
is the manager code:
from django.db import connection, backend, models
from django.core.exceptions import ObjectDoesNotExist
import sys
class HierarchyManager(models.Manager):
def get_orphans(self, field_name, exclude_list=None):
reln = self.model._meta.get_field(field_name)
to_model_type = reln.rel.to
id_field = backend.quote_name('id')
model_table = backend.quote_name(to_model_type._meta.db_table)
join_table = backend.quote_name(reln.m2m_db_table())
#from_field = backend.quote_name(reln.m2m_column_name())
to_field = backend.quote_name(reln.m2m_reverse_name())
query = """
SELECT %s.%s FROM %s
LEFT JOIN %s ON
%s.%s = %s.%s
WHERE
%s.%s IS NULL""" % (
model_table, id_field, model_table,
join_table,
model_table, id_field, join_table, to_field,
join_table, to_field, )
if exclude_list:
query += """
AND NOT %s.%s IN (%s)
""" % ( model_table, id_field, ",".join([str(item) for item in
exclude_list]) )
cursor = connection.cursor()
cursor.execute(query)
orphan_ids = [item[0] for item in cursor.fetchall()]
return self.filter(id__in = orphan_ids)
This seems to be working quite well, and stays generic enough to use
for any M2M non-symmetrical field for a given model. Any thoughts,
comments, criticisms?
Thanks,
Christian
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