examples.dogpile_caching.local_session_caching — SQLAlchemy 2.0 Documentation

Release: 2.0.39 | Release Date: March 11, 2025

# SQLAlchemy 2.0 Documentation

# Source code for examples.dogpile_caching.local_session_caching

    """This example creates a new dogpile.cache backend that will persist data in a
    dictionary which is local to the current session.   remove() the session and
    the cache is gone.

    Create a new Dogpile cache backend that will store
    cached data local to the current Session.

    This is an advanced example which assumes familiarity
    with the basic operation of CachingQuery.

    """

    from dogpile.cache import make_region
    from dogpile.cache.api import CacheBackend
    from dogpile.cache.api import NO_VALUE
    from dogpile.cache.region import register_backend

    from sqlalchemy import select
    from . import environment
    from .caching_query import FromCache
    from .environment import regions
    from .environment import Session

    class ScopedSessionBackend(CacheBackend):
        """A dogpile backend which will cache objects locally on
        the current session.

        When used with the query_cache system, the effect is that the objects
        in the cache are the same as that within the session - the merge()
        is a formality that doesn't actually create a second instance.
        This makes it safe to use for updates of data from an identity
        perspective (still not ideal for deletes though).

        When the session is removed, the cache is gone too, so the cache
        is automatically disposed upon session.remove().

        """

        def __init__(self, arguments):
            self.scoped_session = arguments["scoped_session"]

        def get(self, key):
            return self._cache_dictionary.get(key, NO_VALUE)

        def set(self, key, value):
            self._cache_dictionary[key] = value

        def delete(self, key):
            self._cache_dictionary.pop(key, None)

        @property
        def _cache_dictionary(self):
            """Return the cache dictionary linked to the current Session."""

            sess = self.scoped_session()
            try:
                cache_dict = sess._cache_dictionary
            except AttributeError:
                sess._cache_dictionary = cache_dict = {}
            return cache_dict

    register_backend("sqlalchemy.session", __name__, "ScopedSessionBackend")

    if __name__ == "__main__":
        # set up a region based on the ScopedSessionBackend,
        # pointing to the scoped_session declared in the example
        # environment.
        regions["local_session"] = make_region().configure(
            "sqlalchemy.session", arguments={"scoped_session": Session}
        )

        from .model import Person

        # query to load Person by name, with criterion
        # of "person 10"
        q = (
            select(Person)
            .filter(Person.name == "person 10")
            .options(FromCache("local_session"))
        )

        # load from DB
        person10 = Session.scalars(q).one()

        # next call, the query is cached.
        person10 = Session.scalars(q).one()

        # clear out the Session.  The "_cache_dictionary" dictionary
        # disappears with it.
        Session.remove()

        # query calls from DB again
        person10 = Session.scalars(q).one()

        # identity is preserved - person10 is the *same* object that's
        # ultimately inside the cache.   So it is safe to manipulate
        # the not-queried-for attributes of objects when using such a
        # cache without the need to invalidate - however, any change
        # that would change the results of a cached query, such as
        # inserts, deletes, or modification to attributes that are
        # part of query criterion, still require careful invalidation.
        cache_key = FromCache("local_session")._generate_cache_key(
            q, {}, environment.cache
        )

        assert person10 is regions["local_session"].get(cache_key)().scalar()