=========================
Defining the Domain Model
=========================

The first change we'll make to our stock pcreate-generated application will
be to define a :term:`domain model` constructor representing a wiki page.
We'll do this inside our ``models.py`` file.

The source code for this tutorial stage can be browsed at
`http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki2/src/models/
<http://github.com/Pylons/pyramid/tree/master/docs/tutorials/wiki2/src/models/>`_.

Making Edits to ``models.py``
-----------------------------

.. note::

  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.

Here's what our ``models.py`` file should look like after this step:

.. literalinclude:: src/models/tutorial/models.py
   :linenos:
   :language: py

The first thing we've done is to do is remove the stock ``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 added a ``Page`` class.  Because this is a SQLAlchemy application,
this class inherits from an instance of
:class:`sqlalchemy.ext.declarative.declarative_base`.

.. literalinclude:: src/models/tutorial/models.py
   :pyobject: Page
   :linenos:
   :language: python

As you can see, our ``Page`` class has a class level attribute
``__tablename__`` which equals the string ``'pages'``.  This means that
SQLAlchemy will store our wiki data in a SQL table named ``pages``.  Our Page
class will also have class-level attributes named ``id``, ``name`` and
``data`` (all instances of :class:`sqlalchemy.Column`).  These will map to
columns in the ``pages`` table.  The ``id`` attribute will be the primary key
in the table.  The ``name`` attribute will be a text attribute, each value of
which needs to be unique within the column.  The ``data`` attribute is a text
attribute that will hold the body of each page.

Changing ``scripts/populate.py``
--------------------------------

We haven't looked at the guts of this file yet, but within the ``scripts``
directory of your ``tutorial`` package is a file named ``populate.py``.  Code
in this file is executed whenever we run the ``populate_tutorial`` command
(as we did in the installation step of this tutorial).

Since we've changed our model, we need to make changes to our ``populate.py``
script.  In particular, we'll replace our import of ``MyModel`` with one of
``Page`` and we'll change the very end of the script to create a ``Page``
rather than a ``MyModel`` and add it to our ``DBSession``.

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

.. literalinclude:: src/models/tutorial/models.py
   :linenos:
   :language: python

Repopulating the Database
-------------------------

Because our model has changed, in order to repopulate the database, we need
to rerun the ``populate_tutorial`` command to pick up the changes you've made
to both the models.py file and to the populate.py file.  From the root of the
``tutorial`` project, directory execute the following commands.

On UNIX:

.. code-block:: text

   $ ../bin/populate_tutorial development.ini

On Windows:

.. code-block:: text

   c:\pyramidtut\tutorial> ..\Scripts\populate_tutorial development.ini

Success will look something like this::

  2011-11-27 01:22:45,277 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                PRAGMA table_info("pages")
  2011-11-27 01:22:45,277 INFO  [sqlalchemy.engine.base.Engine][MainThread] ()
  2011-11-27 01:22:45,277 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
  CREATE TABLE pages (
  	id INTEGER NOT NULL, 
  	name TEXT, 
  	data TEXT, 
  	PRIMARY KEY (id), 
  	UNIQUE (name)
  )


  2011-11-27 01:22:45,278 INFO  [sqlalchemy.engine.base.Engine][MainThread] ()
  2011-11-27 01:22:45,397 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                COMMIT
  2011-11-27 01:22:45,400 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                BEGIN (implicit)
  2011-11-27 01:22:45,401 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                INSERT INTO pages (name, data) VALUES (?, ?)
  2011-11-27 01:22:45,401 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                ('FrontPage', 'This is the front page')
  2011-11-27 01:22:45,402 INFO  [sqlalchemy.engine.base.Engine][MainThread] 
                                COMMIT

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:

.. code-block:: text

   ImportError: cannot import name MyModel

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