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==================================
26: Storing Resources In Databases
==================================
Store and retrieve resource tree containers and items in a database.
Background
==========
We now have a resource tree that can go infinitely deep,
adding items and subcontainers along the way. We obviously need a
database, one that can support hierarchies. ZODB is a transaction-based
Python database that supports transparent persistence. We will modify
our application to work with the ZODB.
Along the way we will add the use of ``pyramid_tm``,
a system for adding transaction awareness to our code. With this we
don't need to manually manage our transaction begin/commit cycles in
our application code. Instead, transactions are setup transparently on
request/response boundaries, outside our application code.
Objectives
==========
- Create a CRUD app that adds records to persistent storage.
- Setup ``pyramid_tm`` and ``pyramid_zodbconn``.
- Make our "content" classes inherit from ``Persistent``.
- Set up a database connection string in our application.
- Set up a root factory that serves the root from ZODB rather than from
memory.
Steps
=====
#. We are going to use the previous step as our starting point:
.. code-block:: bash
(env27)$ cd ..; cp -r traversal_addcontent traversal_zodb; cd traversal_zodb
#. Introduce some new dependencies in ``traversal_zodb/setup.py``:
.. literalinclude:: traversal_zodb/setup.py
:linenos:
#. We can now install our project:
.. code-block:: bash
(env27)$ python setup.py develop
#. Modify our ``traversal_zodb/development.ini`` to include some
configuration and give database connection parameters:
.. literalinclude:: traversal_zodb/development.ini
:language: ini
:linenos:
#. Our startup code in ``traversal_zodb/tutorial/__init__.py`` gets
some bootstrapping changes:
.. literalinclude:: traversal_zodb/tutorial/__init__.py
:linenos:
#. Our views in ``traversal_zodb/tutorial/views.py`` change to create
persistent objects:
.. literalinclude:: traversal_zodb/tutorial/views.py
:linenos:
#. As do our resources in ``traversal_zodb/tutorial/resources.py``:
.. literalinclude:: traversal_zodb/tutorial/resources.py
:linenos:
#. Run your Pyramid application with:
.. code-block:: bash
(env27)$ pserve development.ini --reload
#. Open ``http://localhost:6543/`` in your browser.
Analysis
========
We install ``pyramid_zodbconn`` to handle database connections to ZODB. This
pulls the ZODB3 package as well.
To enable ``pyramid_zodbconn``:
- We activate the package configuration using ``pyramid.includes``.
- We define a ``zodbconn.uri`` setting with the path to the Data.fs file.
In the root factory, instead of using our old root object, we now get a
connection to the ZODB and create the object using that.
Our resources need a couple of small changes. Folders now inherit from
persistent.PersistentMapping and document from persistent.Persistent. Note
that Folder now needs to call super() on the __init__ method, or the
mapping will not initialize properly.
On the bootstrap, note the use of transaction.commit() to commit the
change. This is because, on first startup, we want a root resource in
place before continuing.
Extra Credit
============
#. Create a view that deletes a document.
#. Remove the configuration line that includes ``pyramid_tm``. What
happens when you restart the application? Are your changes
persisted across restarts?
#. What happens if you delete the files named ``Data.fs*``?
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