code: def within_radius_sphere(self, x, y, radius):
          
          return QueryExpression({
              self : {'$within' : {
                      '$centerSphere' : [[x, y], radius],
                  }}
              })
docs: Adapted from the Mongo docs::
  
                  session.query(Places).filter(Places.loc.within_radius_sphere(1, 2, 50)