Peter Hudec Peter Hudec - 2 months ago 12
Python Question

How to Clear/Invalidate NDB Cache in Tests

I have a simple NDB Model with an instance attribute, which is not an NDB property. I want to test that the property value gets lost when the entity is saved and retrieved from the datastore.

Now when the entity is retrieved it is the same instance as the one that has been put to the datastore.

Is there a way to programatically clear/invalidate the memcache and the NDB in-memory cache?

The example is a pytest test, sorry for that.

from google.appengine.ext import testbed, ndb


class User(ndb.Model):
name = ndb.TextProperty()
foo = None


class TestNDBModel(object):

def setup(self):
self.testbed = testbed.Testbed()
self.testbed.activate()
self.testbed.init_datastore_v3_stub()
self.testbed.init_memcache_stub()

def teardown(self):
self.testbed.deactivate()

def test_foo(self):
user = User(name='Andy')
assert user.name == 'Andy'
assert user.foo is None
user.foo = 123
assert user.foo == 123
user_key = user.put()

# Here I need to clear the cache somehow,
# so that the entity must be retrieved from datastore

retrieved_user = user_key.get()

# These two will now fail
assert retrieved_user is not user
assert retrieved_user.foo is None

Answer

The solution is to prevent an entity to be cached when it is being put to the DB with entity.put(use_cache=False, use_memcache=False).

from google.appengine.ext import testbed, ndb


class User(ndb.Model):
    name = ndb.TextProperty()
    foo = None


class TestNDBModel(object):

    def setup(self):
        self.testbed = testbed.Testbed()
        self.testbed.activate()
        self.testbed.init_datastore_v3_stub()
        self.testbed.init_memcache_stub()

    def teardown(self):
        self.testbed.deactivate()

    def test_foo(self):
        user = User(name='Andy')
        assert user.name == 'Andy'
        assert user.foo is None
        user.foo = 123
        assert user.foo == 123

        # This prevents the entity to be cached
        user_key = user.put(use_cache=False, use_memcache=False)

        # The entity is now retrieved from DB
        retrieved_user = user_key.get()

        # These two will now pass
        assert retrieved_user is not user
        assert retrieved_user.foo is None