Class MigrationRecorder

django/db/migrations/recorder.py:9–111

Deal with storing migration records in the database. Because this table is actually itself used for dealing with model creation, it's the one thing we can't do normally via migrations. We manually handle table creation/schema updating (using schema backend) and then have a floa

class MigrationRecorder:
    """
    Deal with storing migration records in the database.

    Because this table is actually itself used for dealing with model
    creation, it's the one thing we can't do normally via migrations.
    We manually handle table creation/schema updating (using schema backend)
    and then have a floating model to do queries with.

    If a migration is unapplied its row is removed from the table. Having
    a row in the table always means a migration is applied.
    """

    _migration_class = None

    @classproperty
    def Migration(cls):
        """
        Lazy load to avoid AppRegistryNotReady if installed apps import
        MigrationRecorder.
        """
        if cls._migration_class is None:

            class Migration(models.Model):
                app = models.CharField(max_length=255)
                name = models.CharField(max_length=255)
                applied = models.DateTimeField(default=now)

                class Meta:
                    apps = Apps()
                    app_label = "migrations"
                    db_table = "django_migrations"

                def __str__(self):
                    return "Migration %s for %s" % (self.name, self.app)

            cls._migration_class = Migration
        return cls._migration_class

    def __init__(self, connection):
        self.connection = connection
        self._has_table = False

    @property
    def migration_qs(self):
        return self.Migration.objects.using(self.connection.alias)

    def has_table(self):
        """Return True if the django_migrations table exists."""
        # If the migrations table has already been confirmed to exist, don't
        # recheck it's existence.
        if self._has_table:
            return True
        # It hasn't been confirmed to exist, recheck.
        with self.connection.cursor() as cursor:
            tables = self.connection.introspection.table_names(cursor)

        self._has_table = self.Migration._meta.db_table in tables