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Advanced Models

Pylons works well with many different types of databases, in addition to other database object-relational mappers.

Advanced SQLAlchemy

Alternative SQLAlchemy Styles

In addition to the declarative style, SQLAlchemy has a default more verbose and explicit approach.

Definitions using the default SQLAlchemy approach

Here is a sample model/__init__.py with a “persons” table, based on the default SQLAlchemy approach:

"""The application's model objects"""
import sqlalchemy as sa
from sqlalchemy import orm

from myapp.model import meta

def init_model(engine):
    meta.Session.configure(bind=engine)
    meta.engine = engine


t_persons = sa.Table("persons", meta.metadata,
    sa.Column("id", sa.types.Integer, primary_key=True),
    sa.Column("name", sa.types.String(100), primary_key=True),
    sa.Column("email", sa.types.String(100)),
    )

class Person(object):
    pass

orm.mapper(Person, t_persons)

This model has one table, “persons”, assigned to the variable t_persons. Person is an ORM class which is bound to the table via the mapper.

Relation example

Here’s an example of a Person and an Address class with a many:many relationship on people.my_addresses. See Relational Databases for People in a Hurry and the `SQLAlchemy manual`_ for details.

t_people = sa.Table('people', meta.metadata,
    sa.Column('id', sa.types.Integer, primary_key=True),
    sa.Column('name', sa.types.String(100)),
    sa.Column('email', sa.types.String(100)),
    )

t_addresses_people = sa.Table('addresses_people', meta.metadata,
    sa.Column('id', sa.types.Integer, primary_key=True),
    sa.Column('person_id', sa.types.Integer, sa.ForeignKey('people.id')),
    sa.Column('address_id', sa.types.Integer, sa.ForeignKey('addresses.id')),
    )

t_addresses = sa.Table('addresses', meta.metadata,
    sa.Column('id', sa.types.Integer, primary_key=True),
    sa.Column('address', sa.types.String(100)),
    )

class Person(object):
    pass

class Address(object):
    pass

orm.mapper(Address, t_addresses)
orm.mapper(Person, t_people, properties = {
    'my_addresses' : orm.relation(Address, secondary = t_addresses_people),
    })

Definitions using “reflection” of an existing database table

If the table already exists, SQLAlchemy can read the column definitions directly from the database. This is called reflecting the table.

The advantage of this approach is that it allows you to dispense with the task of specifying the column types in Python code.

Reflecting existing database tables must be done inside init_model() because to perform the reflection, a live database engine is required and this is not available when the module is imported. A live database engine is bound explicitly in the init_model() function and so enables reflection.

(An engine is a SQLAlchemy object that knows how to connect to a particular database.)

Here’s the second example with reflection:

"""The application's model objects"""
import sqlalchemy as sa
from sqlalchemy import orm

from myapp.model import meta

def init_model(engine):
    """Call me before using any of the tables or classes in the model"""
    # Reflected tables must be defined and mapped here
    global t_persons
    t_persons = sa.Table("persons", meta.metadata, autoload=True,
                         autoload_with=engine)
    orm.mapper(Person, t_persons)

    meta.Session.configure(bind=engine)
    meta.engine = engine


t_persons = None

class Person(object):
    pass

Note how t_persons and the orm.mapper() call moved into init_model(), while the Person class didn’t have to. Also note the global t_persons statement. This tells Python that t_persons is a global variable outside the function. global is required when assigning to a global variable inside a function. It’s not required if you’re merely modifying a mutable object in place, which is why meta doesn’t have to be declared global.

Using the model standalone

You now have everything necessary to use the model in a standalone script such as a cron job, or to test it interactively. You just need to create a SQLAlchemy engine and connect it to the model. This example uses a database “test.sqlite” in the current directory:

% python
Python 2.5.1 (r251:54863, Oct 5 2007, 13:36:32)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlalchemy as sa
>>> engine = sa.create_engine("sqlite:///test.sqlite")
>>> from myapp import model
>>> model.init_model(engine)

Now you can use the tables, classes, and Session as described in the `SQLAlchemy manual`_. For example:

#!/usr/bin/env python
import sqlalchemy as sa
import tmpapp.model as model
import tmpapp.model.meta as meta

DB_URL = "sqlite:///test.sqlite"

engine = sa.create_engine(DB_URL)
model.init_model(engine)

# Create all tables, overwriting them if they exist.
if hasattr(model, "_Base"):
    # SQLAlchemy 0.5 Declarative syntax
    model._Base.metadata.drop_all(bind=engine, checkfirst=True)
    model._Base.metadata.create_all(bind=engine)
else:
    # SQLAlchemy 0.4 and 0.5 syntax without Declarative
    meta.metadata.drop_all(bind=engine, checkfirst=True)
    meta.metadataa.create_all(bind=engine)

# Create two records and insert them into the database using the ORM.
a = model.Person()
a.name = "Aaa"
a.email = "aaa@example.com"
meta.Session.add(a)

b = model.Person()
b.name = "Bbb"
b.email = "bbb@example.com"
meta.Session.add(b)

meta.Session.commit()

# Display all records in the persons table.
print "Database data:"
for p in meta.Session.query(model.Person):
    print "id:", p.id
    print "name:", p.name
    print "email:", p.email
    print

Talking to Multiple Databases at Once

Some applications need to connect to multiple databases (engines). Some always bind certain tables to the same engines (e.g., a general database and a logging database); this is called “horizontal partitioning”. Other applications have several databases with the same structure, and choose one or another depending on the current request. A blogging app with a separate database for each blog, for instance. A few large applications store different records from the same logical table in different databases to prevent the database size from getting too large; this is called “vertical partitioning” or “sharding”. The pattern above can accommodate any of these schemes with a few minor changes.

First, you can define multiple engines in your config file like this:

sqlalchemy.default.url = "mysql://..."
sqlalchemy.default.pool_recycle = 3600
sqlalchemy.log.url = "sqlite://..."

This defines two engines, “default” and “log”, each with its own set of options. Now you have to instantiate every engine you want to use.

default_engine = engine_from_config(config, 'sqlalchemy.default.')
log_engine = engine_from_config(config, 'sqlalchemy.log.')
init_model(default_engine, log_engine)

Of course you’ll have to modify init_model() to accept both arguments and create two engines.

To bind different tables to different databases, but always with a particular table going to the same engine, use the binds argument to sessionmaker rather than bind:

binds = {"table1": engine1, "table2": engine2}
Session = scoped_session(sessionmaker(binds=binds))

To choose the bindings on a per-request basis, skip the sessionmaker bind(s) argument, and instead put this in your base controller’s __call__ method before the superclass call, or directly in a specific action method:

meta.Session.configure(bind=meta.engine)

binds= works the same way here too.

Multiple Application Instances

If you’re running multiple instances of the _same_ Pylons application in the same WSGI process (e.g., with Paste HTTPServer’s “composite” application), you may run into concurrency issues. The problem is that Session is thread local but not application-instance local. We’re not sure how much this is really an issue if Session.remove() is properly called in the base controller, but just in case it becomes an issue, here are possible remedies:

  1. Attach the engine(s) to pylons.g (aka. config["pylons.g"]) rather than to the meta module. The globals object is not shared between application instances.
  2. Add a scoping function. This prevents the application instances from sharing the same session objects. Add the following function to your model, and pass it as the second argument to scoped_session:
def pylons_scope():
    import thread
    from pylons import config
    return "Pylons|%s|%s" % (thread.get_ident(), config._current_obj())

Session = scoped_session(sessionmaker(), pylons_scope)

If you’re affected by this, or think you might be, please bring it up on the pylons-discuss mailing list. We need feedback from actual users in this situation to verify that our advice is correct.

Non-SQLAlchemy libraries

Most of these expose only the object-relational mapper; their SQL builder and connection pool are not meant to be used directly.

Storm

Geniusql

DB-API

All the SQL libraries above are built on top of Python’s DB-API, which provides a common low-level interface for interacting with several database engines: MySQL, PostgreSQL, SQLite, Oracle, Firebird, MS-SQL, Access via ODBC, etc. Most programmers do not use DB-API directly because its API is low-level and repetitive and does not provide a connection pool. There’s no “DB-API package” to install because it’s an abstract interface rather than software. Instead, install the Python package for the particular engine you’re interested in. Python’s Database Topic Guide describes the DB-API and lists the package required for each engine. The sqlite3 package for SQLite is included in Python 2.5.

Object Databases

Object databases store Python dicts, lists, and classes in pickles, allowing you to access hierarchical data using normal Python statements rather than having to map them to tables, relations, and a foreign language (SQL).

ZODB

Durus [1]

[1]Durus is not thread safe, so you should use its server mode if your application writes to the database. Do not share connections between threads. ZODB is thread safe, so it may be a more convenient alternative.