How do I check if object exist in pony orm cache? - python-3.6

Question

I am trying some data processing and then insert in one go, tables have some composite keys which I use to check if the record for that Id exists then it should update the record instead of creating.
While data processing there is a chance that same id exists in sample data multiple times and while processing it won't find record matching to that Id in db hence it tries to create the record everytime.
Is there any way I can check cache for composite key match?

Answer 1

You can use a get method of entity instance to find an object by combination of primary key values.
The get method searches both db_session cache and database, and returns None if no object was found:
from pony import orm
db = orm.Database('sqlite', ':memory:')
class Point(db.Entity):
x = orm.Required(int)
y = orm.Required(int)
description = orm.Optional(str)
orm.PrimaryKey(x, y)
db.generate_mapping(create_tables=True)
points = [(1, 2, 'foo'), (3, 4, 'bar'), (1, 2, 'baz'), (5, 6, 'qux')]
with orm.db_session:
for a, b, s in points:
point = Point.get(x=a, y=b)
if point is None:
point = Point(x=a, y=b, description=s)
The get method also works with secondary composite keys.
Also you can search for an entity instance using square brackets and catch an exception:
with orm.db_session:
try:
point = Point[10, 20]
except orm.ObjectNotFound:
point = Point(x=10, y=20)
Update: clarification about IdentityMap patern usage
PonyORM uses IdentityMap design pattern. That means that all objects of the same class loaded or created inside a db_session are indexed by its primary key values. It is not possible multiple objects of the same class with the same primary key inside the same db_session. If the get method is called several times with the same primary key value, it will return the same instance:
with db_session:
a = Point.get(x=10, y=20)
b = Point.get(x=10, y=20)
assert a is b # the same object!
The same applied to situation when you just create an object and then attempts to get object with the same primary key: Pony will return the object that you just created:
with db_session:
a = Point.get(x=10, y=20)
if a is None:
a = Point(x=10, y=20)
# later, in the same db_session:
b = Point.get(x=10, y=20)
assert a is b # the same object, not saved yet
On the other side, if you attempts to create two different objects with the same primary key, you will receive an error:
with db_session:
a = Point(x=10, y=20)
b = Point(x=10, y=20)
Traceback (most recent call last):
...
pony.orm.core.CacheIndexError: Cannot create Point: instance with primary key 10, 20 already exists
Also, if you attempt to create an object which already exists in the database without checking it, you will get an error:
with db_session:
a = Point(x=30, y=40)
with db_session:
b = Point(x=30, y=40)
Traceback (most recent call last):
...
pony.orm.core.TransactionIntegrityError: Object Point[30, 40] cannot be stored in the database. IntegrityError: UNIQUE constraint failed: Point.x, Point.y
In order to avoid this error, you can check for object existense using get before creating a new one.