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Models
======
*Models* are typically simple Python classes defined in a module.
Model *instances* make up the graph that ``repoze.bfg`` is willing to
traverse.
Defining a Model
----------------
An example of a model describing a blog entry::
import datetime
from zope.interface import implements
from zope.interface import Interface
class IBlogEntry(Interface):
pass
class BlogEntry(object):
implements(IBlogEntry)
def __init__(self, title, body, author):
self.title = title
self.body = body
self.author = author
self.created = datetime.datetime.now()
A model consists of basically two things: the object which defines the
model (above as the class ``IBlogEntry``), and an *interface* attached
to the model object. An interface simply tags the model object with a
"type" that can be referred to within view configuration. A model
object can implement zero or more interfaces.
Defining a Graph of Model Instances
-----------------------------------
``repoze.bfg`` expects to be able to traverse a graph of model
instances. ``repoze.bfg`` imposes the following policy on model
instance nodes in the graph:
- Nodes which contain other nodes (aka "container" nodes) must supply
a ``__getitem__`` method which is willing to resolve a string or
unicode name to a subobject. If a subobject by that name does not
exist in the container, ``__getitem__`` must raise a KeyError. If
a subobject by that name *does* exist, the container should return
the subobject (another model instance).
- Nodes which do not contain other nodes (aka "leaf" nodes) must not
implement ``__getitem__``.
Location-Aware Model Instances
------------------------------
- For ``repoze.bfg`` security and convenience URL-generation
functions to work properly against a model instance graph, all
nodes in the graph should have two attributes:: ``__parent__`` and
``__name__``. The ``__parent__`` attribute should be a reference
to the node's parent model instance in the graph. The ``__name__``
attribute should be the name that a node's parent refers to the
node by via ``__getitem__``.
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