gistfile1.txt

nav = SiteNavigation(site)
nav.as_dict()

nav = SiteNavigation.for_site_id(site_id)
nav.as_dict()

models.py

class SiteNavigation(object):
  def __init__(self, site):
    """Initialize a nav object with a site OR a site id"""
    if not isinstance(site, Site):
      site = Site.objects.get(site)
    # perform init...

# vs...

class SiteNavigation(object):
  def __init__(self, site):
    """Initialize a nav object with a site"""
    # perform init...
  
  @classmethod
  def for_site_id(cls, site_id):
    """Initialize a nav object with a site id"""
    site = Site.objects.get(site_id)
    return cls(site)