deb-python-pecan/docs/source/databases.rst

Working with Databases and ORM's ============= Out of the box, Pecan provides no opinionated support for working with databases, but it's easy to hook into your ORM of choice with minimal effort. This article details best practices for integration the popular Python ORM, SQLAlchemy, into your Pecan project.

init_model and Preparing Your Model ----------------Pecan's default quickstart project includes an empty stub directory for implementing your model as you see fit:

.

└── test_project

├── app.py ├── __init__.py ├── controllers ├── model │   ├── __init__.py └── templates

By default, this module contains a special method, init_model:

from pecan import conf

def init_model():
    # Read and parse database bindings from pecan.conf
    pass

The purpose of this method is to determine bindings from your configuration file and create necessary engines, pools, etc... according to your ORM or database toolkit of choice.

Additionally, your project's model module can be used to define functions for common binding operations, such as starting transactions, committing or rolling back work, and clearing a Session. This is also the location in your project where object and relation definitions should be defined. Here's what a sample Pecan configuration file with database bindings might look like:

# Server Specific Configurations
server = {
    ...
}

# Pecan Application Configurations
app = {
    ...
}

# Bindings and options to pass to SQLAlchemy's ``create_engine``
sqlalchemy = {
    'url'           : 'mysql://root:@localhost/atrium?charset=utf8&use_unicode=0',
    'echo'          : False,
    'echo_pool'     : False,
    'pool_recycle'  : 3600,
    'encoding'      : 'utf-8'
}

...and a basic model implementation that can be used to configure and bind using SQLAlchemy:

from pecan                  import conf
from sqlalchemy             import create_engine, MetaData
from sqlalchemy.orm         import scoped_session, sessionmaker

Session = scoped_session(sessionmaker())
metadata = MetaData()

def _engine_from_config(configuration):
    configuration = dict(configuration)
    url = configuration.pop('url')
    return create_engine(url, **configuration)

def init_model():
    conf.sqlalchemy.engine = _engine_from_config(conf.sqlalchemy)

def start():
    Session.bind = conf.sqlalchemy.engine
    metadata.bind = Session.bind

def commit():
    Session.commit()

def rollback():
    Session.rollback()

def clear():
    Session.remove()

Binding Within the Application ----------------There are several approaches that can be taken to wrap your application's requests with calls to appropriate model function calls. One approach is WSGI middleware. We also recommend Pecan hooks (see ref:hooks). Pecan comes with TransactionHook, a hook which can be used to wrap requests in transactions for you. To use it, you simply include it in your project's app.py file and pass it a set of functions related to database binding:

app = make_app(
    conf.app.root,
    static_root     = conf.app.static_root,
    template_path   = conf.app.template_path,
    debug           = conf.app.debug,
    hooks           = [
        TransactionHook(
            model.start,
            model.start_read_only,
            model.commit,
            model.rollback,
            model.clear
        )
    ]
)

For the above example, on HTTP POST, PUT, and DELETE requests, TransactionHook behaves in the following manner:

#.  Before controller routing has been determined, ``model.start()`` is called.  This function

should bind to the appropriate SQLAlchemy engine and start a transaction. #. Controller code is run and returns. #. If your controller or template rendering fails and raises an exception, model.rollback() is called and the original exception is re-raised. This allows you to rollback your database transaction to avoid committing work when exceptions occur in your application code. #. If the controller returns successfully, model.commit() and model.clear() are called.

On idempotent operations (like HTTP GET and HEAD requests), TransactionHook behaves in the following manner:

#.  ``model.start_read_only()`` is called.  This function should bind to your SQLAlchemy engine.
#.  Controller code is run and returns.
#.  If the controller returns successfully, ``model.clear()`` is called.

Splitting Reads and Writes ----------------Employing the above strategy with TransactionHook makes it very simple to split database reads and writes based upon HTTP methods (i.e., GET/HEAD requests are read and would potentially be routed to a read-only database slave, while POST/PUT/DELETE requests require writing, and would bind to a master database with read/write priveleges) It's also very easy extend TransactionHook or write your own hook implementation for more refined control over where and when database bindings are called.