GeoDjango：如何根据点和半径创建圆

class Zone(gismodels.Model):
    name = gismodels.CharField()
    poly = gismodels.PolygonField()

>>> from django.contrib.gis import geos
>>> center = geos.Point(5, 5)
>>> radius = 2
>>> circle = center.buffer(radius)
>>> circle
<Polygon object at 0x1029d8370>