Working with Databases, Transactions, and ORM’s

Pecan provides no opinionated support for working with databases, but it’s easy to hook into your ORM of choice. This article details best practices for integrating 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():
    """
    This is a stub method which is called at application startup time.

    If you need to bind to a parsed database configuration, set up tables
    or ORM classes, or perform any database initialization, this is the
    recommended place to do it.

    For more information working with databases, and some common recipes,
    see http://pecan.readthedocs.org/en/latest/databases.html
    """
    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/dbname?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 to wrapping your application’s requests with calls to appropriate model function calls. One approach is WSGI middleware. We also recommend Pecan Pecan Hooks. Pecan comes with TransactionHook, a hook which can be used to wrap requests in database transactions for you. To use it, simply include it in your project’s app.py file and pass it a set of functions related to database binding.

from pecan import conf, make_app
from pecan.hooks import TransactionHook
from test_project import model

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
        )
    ]
)

In the above example, on HTTP POST, PUT, and DELETE requests, TransactionHook takes care of the transaction automatically by following these rules:

1. Before controller routing has been determined, model.start() is called. This function should bind to the appropriate SQLAlchemy engine and start a transaction.
2. Controller code is run and returns.
3. 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.
4. If the controller returns successfully, model.commit() and model.clear() are called.

On idempotent operations (like HTTP GET and HEAD requests), TransactionHook handles transactions following different rules.

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

Also note that there is a useful after_commit() decorator provided in pecan.decorators – Pecan Decorators.

Splitting Reads and Writes

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