Key References for Persistent Objects
=====================================

`zope.keyreference.persistent.KeyReferenceToPersistent` provides an
`zope.app.keyreference.interfaces.IReference` reference for persistent
objects.  

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

    >>> from ZODB.tests.util import DB
    >>> import transaction
    >>> from persistent.dict import PersistentDict

    >>> db = DB()
    >>> conn = db.open()
    >>> root = conn.root()

    >>> root['ob1'] = PersistentDict()
    >>> root['ob2'] = PersistentDict()

    >>> transaction.commit()

Then we'll create some key references:

    >>> from zope.app.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 implements
    >>> from zope.app.keyreference.interfaces import IKeyReference

    >>> class DummyKeyReference(object):
    ...     implements(IKeyReference)
    ...     key_type_id = 'zope.app.keyreference.object'
    ...     def __init__(self, obj):
    ...         self.object = obj
    ...     def __cmp__(self, other):
    ...          if self.key_type_id == other.key_type_id:
    ...              return cmp(self.object, other.object)
    ...          return cmp(self.key_type_id, other.key_type_id)

    >>> dummy_key1 = DummyKeyReference(object())
    >>> dummy_key2 = DummyKeyReference(object())
    >>> dummy_key3 = DummyKeyReference(object())

    >>> 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(PersistentDict())
    ... # doctest: +ELLIPSIS
    Traceback (most recent call last):
    ...
    NotYet: <persistent.dict.PersistentDict object at ...>

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()


Location-based connection adapter
---------------------------------

The function `zope.app.keyreference.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.app.keyreference.persistent import connectionOfPersistent
    >>> ob3 = PersistentDict()
    >>> print connectionOfPersistent(ob3)
    None

    >>> ob3.__parent__ = root2['ob1']
    >>> connectionOfPersistent(ob3) is conn2
    True
