IntegrityError: (1062, "Duplicate entry '1830327-1792993' for key 'some_instance_A_id'") but no UNIQUE constraint

Question

I'm rarely getting such exception when adding model instance to many2many field, i.e.:

 some_instance_A.my_m2m.add(some_instance_B)). 

It works say 99/100 times. What looks strange to me, is that dash sign - as primary keys are integers..

Model field is defined like this:

 my_m2m = ManyToManyField(B)

so it's the simplest M2M definition possible

Full traceback (with minor edits to protect privacy :)):

File "*******", line 278, in process_request
   some_instance_A.my_m2m.add(some_instance_B)
 File "/var/lib/python-support/python2.5/django/db/models/fields/related.py", line 490, in add
   self._add_items(self.source_field_name, self.target_field_name, *objs)
 File "/var/lib/python-support/python2.5/django/db/models/fields/related.py", line 574, in _add_items
   '%s_id' % target_field_name: obj_id,
 File "/var/lib/python-support/python2.5/django/db/models/query.py", line 352, in create
   obj.save(force_insert=True, using=self.db)
 File "/var/lib/python-support/python2.5/django/db/models/base.py", line 435, in save
 self.save_base(using=using, force_insert=force_insert, force_update=force_update)
 File "/var/lib/python-support/python2.5/django/db/models/base.py", line 528, in save_base
 result = manager._insert(values, return_id=update_pk, using=using)
 File "/var/lib/python-support/python2.5/django/db/models/manager.py", line 195, in _insert
 return insert_query(self.model, values, **kwargs)
 File "/var/lib/python-support/python2.5/django/db/models/query.py", line 1479, in insert_query
 return query.get_compiler(using=using).execute_sql(return_id)
 File "/var/lib/python-support/python2.5/django/db/models/sql/compiler.py", line 783, in execute_sql
 cursor = super(SQLInsertCompiler, self).execute_sql(None)
 File "/var/lib/python-support/python2.5/django/db/models/sql/compiler.py", line 727, in execute_sql
 cursor.execute(sql, params)
 File "/var/lib/python-support/python2.5/django/db/backends/mysql/base.py", line 86, in execute
 return self.cursor.execute(query, args)
 File "/var/lib/python-support/python2.5/MySQLdb/cursors.py", line 166, in execute
 self.errorhandler(self, exc, value)
 File "/var/lib/python-support/python2.5/MySQLdb/connections.py", line 35, in defaulterrorhandler
 raise errorclass, errorvalue
 IntegrityError: (1062, "Duplicate entry '1830327-1792993' for key 'some_instance_A_id'")

Answer 1

Can you post the code snippet that yields this result?

At least on Postgres this can happen if you try saving an instance twice inside a transaction.