Count Instance method of Query Class

from google.appengine.ext import db
from google.appengine.api import users
 
class FirstModel(db.Model):
    prop = db.IntegerProperty()

class SecondModel(db.Model):
    prop2 = db.IntegerProperty()
    reference = db.ReferenceProperty(FirstModel, collection_name = "relation")



//Input data to datastore
firstmodel = FirstModel(prop = 06)
firstmodel.put()
secondmodel = SecondModel(prop2 = 1, reference = firstmodel)
secondmodel.put()
secondmodel = SecondModel(prop2 = 2, reference = firstmodel)
secondmodel.put()



//Count
query = firstmodel.relation
query_count = query.count()
print query_count

OR
query = FirstModel.all()
query_count = query.count()
print query_count

Result:
2