How to implement Synchronization Tag in django

Question

We are making a synchronization between one master database and many slave databases over tight link (7kb). To minimize amount of data send we tag each record that was sent successfully.

To do so we created following models:

class SynchronizationTag(models.Model):
    STATUSES = ((0, "Invalid"), 
                (1, "Pending"),
                (2, "Synchronized"), 
                )
    status = models.IntegerField(choices=STATUSES, default = 1)
    storage = models.ForeignKey("Storage")

    _content_type = models.ForeignKey(ContentType)
    _object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('_content_type', '_object_id')

class Storage(models.Model):
   id = models.AutoField(primary_key=True)

To select records that need synchronization we wrote a general query that would filter out records that were tagged as synchronized from a query set.

We came up with following suboptimal solution:

delta = queryset.extra(
 select={ 
  "status" : ("SELECT status FROM rv3adapter_synchronizationtag "
     "WHERE `_content_type_id` = %d and `_object_id`= %s.id and `storage_id` = %d" 
     % (content_type.pk, table, storage.pk)) 
  },
 where=["`status` <> 2 or `status` is NULL")

The query above is now broken as we have few models that don't have primary keys named id.

Do you know how to improve/correct above query?

Note: The query need to return django objects.

Answer 1

Get the name of the primary key field with queryset.model._meta.pk.name.

You might also find ._meta.db_table useful.

Granted, this is using an unofficial API that isn't guaranteed to be compatible in future versions of django, but if you have a few unit tests you should be fine.