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Defining the Domain Model

The first change we’ll make to our stock paster-generated application will be to define two resource constructors, one representing a wiki page, and another representing the wiki as a mapping of wiki page names to page objects. We’ll do this inside our models.py file.

Because we’re using ZODB to represent our resource tree, each of these resource constructors represents a domain model object, so we’ll call these constructors “model constructors”. Both our Page and Wiki constructors will be class objects. A single instance of the “Wiki” class will serve as a container for “Page” objects, which will be instances of the “Page” class.

The source code for this tutorial stage can be browsed via http://github.com/Pylons/pyramid/tree/1.1-branch/docs/tutorials/wiki/src/models/.

Deleting the Database

In the next step, we’re going to remove the MyModel Python model class from our models.py file. Since this class is referred to within our persistent storage (represented on disk as a file named Data.fs), we’ll have strange things happen the next time we want to visit the application in a browser. Remove the Data.fs from the tutorial directory before proceeding any further. It’s always fine to do this as long as you don’t care about the content of the database; the database itself will be recreated as necessary.

Making Edits to models.py


There is nothing automagically special about the filename models.py. A project may have many models throughout its codebase in arbitrarily-named files. Files implementing models often have model in their filenames, or they may live in a Python subpackage of your application package named models, but this is only by convention.

The first thing we want to do is remove the MyModel class from the generated models.py file. The MyModel class is only a sample and we’re not going to use it.

Then, we’ll add a Wiki class. We want it to inherit from the persistent.mapping.PersistentMapping class because it provides mapping behavior, and it makes sure that our Wiki page is stored as a “first-class” persistent object in our ZODB database.

Our Wiki class should have two attributes set to None at class scope: __parent__ and __name__. If a model has a __parent__ attribute of None in a traversal-based Pyramid application, it means that it’s the root model. The __name__ of the root model is also always None.

Then we’ll add a Page class. This class should inherit from the persistent.Persistent class. We’ll also give it an __init__ method that accepts a single parameter named data. This parameter will contain the ReStructuredText body representing the wiki page content. Note that Page objects don’t have an initial __name__ or __parent__ attribute. All objects in a traversal graph must have a __name__ and a __parent__ attribute. We don’t specify these here because both __name__ and __parent__ will be set by by a view function when a Page is added to our Wiki mapping.

As a last step, we want to change the appmaker function in our models.py file so that the root resource of our application is a Wiki instance. We’ll also slot a single page object (the front page) into the Wiki within the appmaker. This will provide traversal a resource tree to work against when it attempts to resolve URLs to resources.

We’re using a mini-framework callable named PersistentApplicationFinder in our application (see __init__.py). A PersistentApplicationFinder accepts a ZODB URL as well as an “appmaker” callback. This callback typically lives in the models.py file. We’ll just change this function, making the necessary edits.

Looking at the Result of Our Edits to models.py

The result of all of our edits to models.py will end up looking something like this:

from persistent import Persistent
from persistent.mapping import PersistentMapping

class Wiki(PersistentMapping):
    __name__ = None
    __parent__ = None

class Page(Persistent):
    def __init__(self, data):
        self.data = data

def appmaker(zodb_root):
    if not 'app_root' in zodb_root:
        app_root = Wiki()
        frontpage = Page('This is the front page')
        app_root['FrontPage'] = frontpage
        frontpage.__name__ = 'FrontPage'
        frontpage.__parent__ = app_root
        zodb_root['app_root'] = app_root
        import transaction
    return zodb_root['app_root']

Viewing the Application in a Browser

We can’t. At this point, our system is in a “non-runnable” state; we’ll need to change view-related files in the next chapter to be able to start the application successfully. If you try to start the application, you’ll wind up with a Python traceback on your console that ends with this exception:

ImportError: cannot import name MyModel

This will also happen if you attempt to run the tests.