Поле, подобно на автоинкремент, за обекти със същия външен ключ (Django 1.8, MySQL 5.5)

В крайна сметка реших проблема, като конструирах и изпълних необработена SQL заявка, за да направя автоматичното увеличаване и вмъкване в една транзакция на база данни. Прекарах много време в изходния код на Django, за да разбера как работи техният метод за запазване на модела по подразбиране, така че да мога да направя това възможно най-стабилно. Въпреки това, напълно очаквам, че това ще трябва да бъде променено за бекендове, различни от MySQL.

Първо, създадох абстрактен клас, от който ObjectLog сега ще произлиза, който включва този нов метод за запазване:

class AutoIncrementModel(models.Model):
    """
    An abstract class used as a base for classes which need the
    autoincrementing save method described below.
    """
    class Meta:
        abstract = True

    def save(self, auto_field, auto_fk, *args, **kwargs):
        """
        Arguments:
            auto_field: name of field which acts as an autoincrement field.
            auto_fk:    name of ForeignKey to which the auto_field is relative.
        """

        # Do normal save if this is not an insert (i.e., the instance has a
        # primary key already).
        meta = self.__class__._meta
        pk_set = self._get_pk_val(meta) is not None
        if pk_set:
            super(ObjectLog, self).save(*args, **kwargs)
            return

        # Otherwise, we'll generate some raw SQL to do the
        # insert and auto-increment.

        # Get model fields, except for primary key field.
        fields = meta.local_concrete_fields
        if not pk_set:
            fields = [f for f in fields if not
                isinstance(f, models.fields.AutoField)]

        # Setup for generating base SQL query for doing an INSERT.
        query = models.sql.InsertQuery(self.__class__._base_manager.model)
        query.insert_values(fields, objs=[self])
        compiler = query.get_compiler(using=self.__class__._base_manager.db)
        compiler.return_id = meta.has_auto_field and not pk_set

        fk_name = meta.get_field(auto_fk).column
        with compiler.connection.cursor() as cursor:
            # Get base SQL query as string.
            for sql, params in compiler.as_sql():
                # compiler.as_sql() looks like:
                # INSERT INTO `table_objectlog` VALUES (%s,...,%s)
                # We modify this to do:
                # INSERT INTO `table_objectlog` SELECT %s,...,%s FROM
                # `table_objectlog` WHERE `object_id`=id
                # NOTE: it's unlikely that the following will generate
                # a functional database query for non-MySQL backends.

                # Replace VALUES (%s, %s, ..., %s) with
                # SELECT %s, %s, ..., %s
                sql = re.sub(r"VALUES \((.*)\)", r"SELECT \1", sql)

                # Add table to SELECT from and ForeignKey id corresponding to
                # our autoincrement field.
                sql += " FROM `{tbl_name}` WHERE `{fk_name}`={fk_id}".format(
                    tbl_name=meta.db_table,
                    fk_name=fk_name,
                    fk_id=getattr(self, fk_name)
                    )

                # Get index corresponding to auto_field.
                af_idx = [f.name for f in fields].index(auto_field)
                # Put this directly in the SQL. If we use parameter
                # substitution with cursor.execute, it gets quoted
                # as a literal, which causes the SQL command to fail.
                # We shouldn't have issues with SQL injection because
                # auto_field should never be a user-defined parameter.
                del params[af_idx]
                sql = re.sub(r"((%s, ){{{0}}})%s".format(af_idx),
                r"\1IFNULL(MAX({af}),0)+1", sql, 1).format(af=auto_field)

                # IFNULL(MAX({af}),0)+1 is the autoincrement SQL command,
                # {af} is substituted as the column name.

                # Execute SQL command.
                cursor.execute(sql, params)

            # Get primary key from database and set it in memory.
            if compiler.connection.features.can_return_id_from_insert:
                id = compiler.connection.ops.fetch_returned_insert_id(cursor)
            else:
                id = compiler.connection.ops.last_insert_id(cursor,
                    meta.db_table, meta.pk.column)
            self._set_pk_val(id)

            # Refresh object in memory in order to get auto_field value.
            self.refresh_from_db()

Тогава моделът ObjectLog използва това като:

class ObjectLog(AutoIncrementModel):
    class Meta:
        ordering = ['-created','-N']
        unique_together = ("object","N")
    object = models.ForeignKey(Object, null=False)                                                                                                                                                              
    created = models.DateTimeField(auto_now_add=True)
    issuer = models.ForeignKey(User)
    N = models.IntegerField(null=False)

    def save(self, *args, **kwargs):
        # Set up to call save method of the base class (AutoIncrementModel)
        kwargs.update({'auto_field': 'N', 'auto_fk': 'event'})
        super(EventLog, self).save(*args, **kwargs)

Това позволява извикванията на ObjectLog.save() да работят според очакванията.