Key References for Persistent Objects

KeyReferenceToPersistent provides an IKeyReference reference for persistent objects.

Let’s look at an example. First, we’ll create some persistent objects in a database:

>>> from ZODB.MappingStorage import DB
>>> import transaction
>>> from persistent.mapping import PersistentMapping
>>> db = DB()
>>> conn = db.open()
>>> root = conn.root()
>>> root['ob1'] = PersistentMapping()
>>> root['ob2'] = PersistentMapping()
>>> transaction.commit()

Then we’ll create some key references:

>>> from zope.keyreference.persistent import KeyReferenceToPersistent
>>> key1 = KeyReferenceToPersistent(root['ob1'])
>>> key2 = KeyReferenceToPersistent(root['ob2'])

We can call the keys to get the objects:

>>> key1() is root['ob1'], key2() is root['ob2']
(True, True)

New keys to the same objects are equal to the old:

>>> KeyReferenceToPersistent(root['ob1']) == key1
True

and have the same hashes:

>>> hash(KeyReferenceToPersistent(root['ob1'])) == hash(key1)
True

Other key reference implementations are differed by their key type id. Key references should sort first on their key type and second on any type-specific information:

>>> from zope.interface import implementer
>>> from zope.keyreference.interfaces import IKeyReference
>>> @implementer(IKeyReference)
... class DummyKeyReference(object):
...     key_type_id = 'zope.app.keyreference.object'
...     def __init__(self, obj):
...         self.object = obj
...     def _get_cmp_keys(self, other):
...          if self.key_type_id == other.key_type_id:
...              return self.object, other.object
...          return self.key_type_id, other.key_type_id
...     def __cmp__(self, other):
...         return cmp(*self._get_cmp_keys(other))
...     def __eq__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a == b
...     def __lt__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a < b
...     def __ne__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a != b
...     def __gt__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a > b
...     def __le__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a <= b
...     def __ge__(self, other):
...         a, b = self._get_cmp_keys(other)
...         return a >= b
>>> dummy_key1 = DummyKeyReference(1)
>>> dummy_key2 = DummyKeyReference(2)
>>> dummy_key3 = DummyKeyReference(3)
>>> keys = [key1, dummy_key1, dummy_key2, key2, dummy_key3]
>>> keys.sort()
>>> key_type_ids = [key.key_type_id for key in keys]
>>> key_type_ids[0:3].count('zope.app.keyreference.object')
3
>>> key_type_ids[3:].count('zope.app.keyreference.persistent')
2

We’ll store the key references in the database:

>>> root['key1'] = key1
>>> root['key2'] = key2

and use the keys to store the objects again:

>>> root[key1] = root['ob1']
>>> root[key2] = root['ob2']
>>> transaction.commit()

Now we’ll open another connection:

>>> conn2 = db.open()

And verify that we can use the keys to look up the objects:

>>> root2 = conn2.root()
>>> key1 = root2['key1']
>>> root2[key1] is root2['ob1']
True
>>> key2 = root2['key2']
>>> root2[key2] is root2['ob2']
True

and that we can also call the keys to get the objects:

>>> key1() is root2['ob1']
True
>>> key2() is root2['ob2']
True

We can’t get the key reference for an object that hasn’t been saved yet:

>>> KeyReferenceToPersistent(PersistentMapping())
... 
Traceback (most recent call last):
...
NotYet: ...

Note that we get a NotYet error. This indicates that we might be able to get a key reference later.

We can get references to unsaved objects if they have an adapter to ZODB.interfaces.IConnection. The add method on the connection will be used to give the object an object id, which is enough information to compute the reference. To see this, we’ll create an object that conforms to IConnection in a silly way:

>>> import persistent
>>> from ZODB.interfaces import IConnection
>>> class C(persistent.Persistent):
...     def __conform__(self, iface):
...         if iface is IConnection:
...             return conn2
>>> ob3 = C()
>>> key3 = KeyReferenceToPersistent(ob3)
>>> transaction.abort()

Conflict Resolution

During conflict resolution, as discussed in ZODB/ConflictResolution.txt, references to persistent objects are actually instances of ZODB.ConflictResolution.PersistentReference. This is pertinent in two ways for KeyReferenceToPersistent. First, it explains a subtlety of the class: it does not inherit from persistent.Persistent. If it did, it would not be available for conflict resolution, just its PersistentReference stand-in.

Second, it explains some of the code in the __hash__ and __cmp__ methods. These methods not only handle persistent.Persistent objects, but PersistentReference objects. Without this behavior, objects, such as the classic ZODB BTrees, that use KeyReferenceToPersistent as keys or set members will be unable to resolve conflicts. Even with the special code, in some cases the KeyReferenceToPersistent will refuse to compare and hash during conflict resolution because it cannot reliably do so.

__hash__ will work relatively rarely during conflict resolution: only for multidatabase references. Here are a couple of examples.

>>> from ZODB.ConflictResolution import PersistentReference
>>> def factory(ref):
...     res = KeyReferenceToPersistent.__new__(
...         KeyReferenceToPersistent, ref)
...     res.object = ref
...     return res
...
>>> hash(factory(PersistentReference(
...     ('an oid', 'class metadata')))) # a typical reference
Traceback (most recent call last):
...
ValueError: database name unavailable at this time
>>> bool(hash(factory(PersistentReference(
...     ['m', ('a database', 'an oid', 'class metadata')])))) # multidatabase
True

This means that KeyReferenceToPersistent will often hinder conflict resolution for classes such as PersistentMapping.

__cmp__ works unless one object is a multidatabase reference and the other is not. Here are a few examples.

>>> def cmp(a, b):
...    return (a > b) - (a < b)
>>> cmp(factory(PersistentReference(
...         (b'an oid', 'class metadata'))),
...     factory(PersistentReference(
...         (b'an oid', 'class metadata'))))
0
>>> cmp(factory(PersistentReference(
...         (b'an oid', 'class metadata'))),
...     factory(PersistentReference(
...         (b'another oid', 'class metadata'))))
-1
>>> cmp(factory(PersistentReference(b'an oid')),
...     factory(PersistentReference(
...         (b'an oid', 'class metadata'))))
0
>>> cmp(factory(PersistentReference(b'an oid')),
...     factory(PersistentReference(
...         (b'an oid', 'class metadata'))))
0
>>> cmp(factory(PersistentReference(
...         ['m', ('a database', b'an oid', 'class metadata')])),
...     factory(PersistentReference(
...         ['m', ('a database', b'an oid', 'class metadata')])))
0
>>> cmp(factory(PersistentReference(
...         ['m', ('a database', b'an oid', 'class metadata')])),
...     factory(PersistentReference(
...         ['n', ('a database', b'an oid')])))
0
>>> cmp(factory(PersistentReference(
...         ['m', ('a database', b'an oid', 'class metadata')])),
...     factory(PersistentReference(
...         ['m', ('another database', b'an oid', 'class metadata')])))
-1
>>> cmp(factory(PersistentReference(
...         ['m', ('a database', b'an oid', 'class metadata')])),
...     factory(PersistentReference(
...         (b'an oid', 'class metadata'))))
Traceback (most recent call last):
...
ValueError: cannot sort reliably

Location-based connection adapter

The function zope.keyreference.persistent.connectionOfPersistent() adapts objects to connections using a simple location-based heuristic. It checked to see if the object has a __parent__ that has a connection:

>>> from zope.keyreference.persistent import connectionOfPersistent
>>> ob3 = PersistentMapping()
>>> print(connectionOfPersistent(ob3))
None
>>> ob3.__parent__ = root2['ob1']
>>> connectionOfPersistent(ob3) is conn2
True