https://github.com/divio/django-cms/issues/1723

base_models.py

class Post(models.Model):
    link = models.URLField(blank=True, null=True, db_index=True) # link is also unique identifier for each entry
    updated = models.DateTimeField(blank=True, null=True, db_index=True)  # last updated
    published = models.DateTimeField(blank=True, null=True, db_index=True) #pubDate
    title = models.CharField(max_length=250, blank=True, null=True) #short title, text without HTML
    description = models.TextField(blank=True, null=True) #universal text/html representation of entry    

class GoscaleCMSPlugin(CMSPlugin):
    """
    Common base abstract class for all the GoScale plugins
    """    
    posts = models.ManyToManyField(Post, verbose_name=_('Posts'))
    template = models.CharField(max_length=255, null=True, blank=True, verbose_name=_('Template'))
    title = models.CharField(max_length=255, null=True, blank=True, verbose_name=_('Title'))
    updated = models.DateTimeField(blank=True, null=True, db_index=True, verbose_name=_('Last updated'))

    class Meta:
        abstract = True

    def copy_relations(self, oldinstance):
        self.posts = oldinstance.posts.all()

models.py

class GooglePresentation(base_models.GoscaleCMSPlugin):
    embed = models.TextField(verbose_name=_('Embed code'), help_text=_('From the "</> Embed" link.'))
    width = models.SmallIntegerField(null=True, blank=True, verbose_name=_('Width'),
        help_text=_('Width of a presentation container.'))
    height = models.SmallIntegerField(null=True, blank=True, verbose_name=_('Height'),
        help_text=_('Height of a presentation container.'))
    ratio = models.CharField(max_length=50,
        default=RATIO_CHOICES[0][0], choices=RATIO_CHOICES, verbose_name=_('Aspect ratio'),
        help_text=_('Ratio of width:height used for the presentation if manual size isn\'t set.'))
    embed_as_is = models.BooleanField(default=False, verbose_name=_('Embed "as is"'),
        help_text=_('If set embed code will not be changed.'))