cromlech.sqlalchemy 0.4

Cromlech Web Framework utility methods and components for applications based on SQLAlchemy.

cromlech.sqlalchemy
###################

``cromlech.sqlalchemy`` provides utility functions and components to ease
the use of `SQLAlchemy`.

Test setup
===========

We get a temporary dir to place our databases::

>>> import tempfile
>>> working_dir = tempfile.mkdtemp('cromlech-sqlalchemy-test')


Configuration & registration
============================

>>> from cromlech.sqlalchemy import create_engine

>>> user_db = 'sqlite:///%s/users.db' % working_dir
>>> store_db = 'sqlite:///%s/store.db' % working_dir

>>> users_engine = create_engine(user_db, 'Users')
>>> store_engine = create_engine(store_db, 'Store')


Model
=====

Let's use declarative extension::

>>> from sqlalchemy.ext.declarative import declarative_base

>>> UserBase = declarative_base()
>>> users_engine.bind(UserBase)

In SQLAlchemy you would declare wich engine the database is bound to. In our
case the url will come in environ with the first connection so we deffer
this to the moment when engine will be created. The package provides a method
for that::


We can define our SQLAlchemy model::

>>> from sqlalchemy import Column, Integer, String

>>> class User(UserBase):
... __tablename__ = 'test_users'
...
... id = Column('id', Integer, primary_key=True)
... name = Column('name', String(50))
...
... def __repr__(self):
... return "User(%d, '%s')" % (self.id, self.name)


Controller
==========

To do any operation we will have use SQLAlchemySession context manager
providing an engine name::

>>> from cromlech.sqlalchemy import SQLAlchemySession, get_session

Typically first request may create our tables::

>>> with SQLAlchemySession(users_engine) as session:
... UserBase.metadata.create_all()
... print session.query(User).all()
[]

Transaction of database is ruled by the general transaction package::

>>> import transaction
>>> transaction.commit()

Inside the context you can also fetch the session by its configuration name,
this is convenient from anywhere in the code, eg a fonction::

>>> def add_user(id, name):
... session = get_session("Users")
... session.add(User(id=id, name=name))

>>> with SQLAlchemySession(users_engine) as session:
... add_user(1, 'bob')
... print session.query(User).all()
[User(1, 'bob')]


Transaction
===========

For this chapter let's assume we are inside a with SQLAlchemySession clause::

>>> ctxmanager = SQLAlchemySession(users_engine)
>>> session = ctxmanager.__enter__()

Transaction are linked to zope transaction using zope.sqlalchemy, all operations
above are not yet commited. Let's abort to see bob is not present no more::

>>> transaction.abort()

>>> print session.query(User).all()
[]

Now add it again an commit so that it's ok::

>>> transaction.commit()
>>> add_user(1, 'bob')
>>> transaction.commit()
>>> print session.query(User).all()
[User(1, 'bob')]

Now we really are in a new transaction::

>>> transaction.abort()
>>> print session.query(User).all()
[User(1, 'bob')]

Let's end our session for now ::

>>> ctxmanager.__exit__(None, None, None)


More than one database
======================

We can use more than one database, Let's define another
model in another database:

>>> StoreBase = declarative_base()
>>> store_engine.bind(StoreBase)

We can define our SQLAlchemy model::

>>> class Product(StoreBase):
... __tablename__ = 'test_products'
... id = Column('id', Integer, primary_key=True)
... name = Column('name', String(50))
...
... def __repr__(self):
... return "Product(%d, '%s')" % (self.id, self.name)


Create tables::

>>> from transaction import manager as transaction_manager

>>> with transaction_manager:
... with SQLAlchemySession(store_engine) as session:
... StoreBase.metadata.create_all()

and an product adder::

>>> def add_product(id, name):
... session = get_session('Store')
... session.add(Product(id=id, name=name))

We may do operations in databases using our objects::

>>> with transaction_manager:
... with SQLAlchemySession(store_engine) as session:
... with SQLAlchemySession(users_engine) as session:
... add_user(2, 'juan')
... add_product(1, 'table')

It works :

>>> with SQLAlchemySession(users_engine) as session:
... session.query(User).all()
[User(1, 'bob'), User(2, 'juan')]

>>> with SQLAlchemySession(store_engine) as session:
... session.query(Product).all()
[Product(1, 'table')]


More than one metadata
======================

Now we may also bind another metadata to same database, and this work
even if it's already initialised::

>>> GroupBase = declarative_base()
>>> users_engine.bind(GroupBase)

>>> class Group(GroupBase):
... __tablename__ = 'test_groups'
... id = Column('id', Integer, primary_key=True)
... name = Column('name', String(50))
...
... def __repr__(self):
... return "Group(%d, '%s')" % (self.id, self.name)

>>> with transaction_manager:
... with SQLAlchemySession(users_engine) as session:
... GroupBase.metadata.create_all()
... session.add(Group(id=1, name='them'))
... session.query(Group).all()
[Group(1, 'them')]


Unique session
==============

Whenever you know that your application will always associate same db name with
same DBName, you can setup session once and only once, removing overhead to
single request. For that use the SharedSQLAlchemySession context manager.
Note that it is still a scoped session (no sharing between thread), and that
you don't have to care when it will be initialized, you just use the context
manager ::

>>> from cromlech.sqlalchemy import SharedSQLAlchemySession
>>> with SharedSQLAlchemySession(store_engine) as session:
... the_session = session
... session.query(Product).all()
[Product(1, 'table')]

session will be reused::

>>> with SharedSQLAlchemySession(store_engine) as session:
... print session is the_session
True


to phase or not two phase
=========================

You can pass a two_phase boolean argument to activate or not two_phase commit.
see zope.sqlalchemy. If you do not pass it, two_phase will be set to true for
mysql and postgre (known supporting database) and false otherwise.

You may force it however::

>>> with transaction_manager:
... with SQLAlchemySession(store_engine, two_phase=True):
... add_product(10, 'chair')
Traceback (most recent call last):
...
ValueError: SQL Engine 'Store' : 'sqlite' does not support two phase commits

as sqlite does not support it !

Changelog
=========

0.4 (2017-03-18)
----------------

- We no longer use the ZCA to register engines. This breaks backward
compatibility by removing the `create_and_register` function.


0.3 (2012-04-29)
----------------

- Majors changes : we no longer support deferred binding. The context
managers now take directly the engine, which is now an `Engine` object.
This makes the implementation much more straightforward.


0.2 (2012-01-26)
----------------

- Fixed testing for py2.7


0.2a1 (2011-07-30)
------------------

- The context manager and util functions no longer depend on an environ.
This allows us to use this package outside of a WSGI stack.


0.1a1
-----

- Initial release