class Album(models.Model):
    thumb = models.ForeignKey('Image',  related_name='image')

class Image(models.Model):
    album = models.ForeignKey('Album', null=True, blank=True)

class AlbumFactory(factory.DjangoModelFactory):
    FACTORY_FOR = 'app.Album'

    thumb = factory.SubFactory(
        'path.to.factories.ImageFactory'
    )

class ImageFactory(factory.DjangoModelFactory):
    FACTORY_FOR = 'app.Image'

    album = factory.RelatedFactory(AlbumFactory, 'thumb')