Bidang seperti peningkatan otomatis untuk objek dengan kunci asing yang sama (Django 1.8, MySQL 5.5)

Saya akhirnya memecahkan masalah dengan membuat dan menjalankan kueri SQL mentah untuk melakukan peningkatan otomatis dan penyisipan dalam satu transaksi basis data. Saya menghabiskan banyak waktu untuk meneliti kode sumber Django untuk memahami bagaimana metode penyimpanan model default mereka bekerja sehingga saya dapat melakukan ini sekuat mungkin. Namun, saya sepenuhnya berharap bahwa ini perlu dimodifikasi untuk backend non-MySQL.

Pertama, saya membuat kelas abstrak yang sekarang akan diturunkan ObjectLog, yang menampilkan metode penyimpanan baru ini:

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()

Kemudian model ObjectLog menggunakan ini seperti:

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)

Ini memungkinkan panggilan ke ObjectLog.save() tetap berfungsi seperti yang diharapkan.