Slides python elixir

SQLAlchemy and Elixir

Jonathan LaCour

http://cleverdevil.org
jonathan@cleverdevil.org

What is SQLAlchemy?

“SQLAlchemy is the Python SQL toolkit and
Object Relational Mapper that gives application
developers the full power and ﬂexibility of SQL.”

What is Elixir?

“Elixir is a declarative layer on top of SQLAlchemy.
It is a thin wrapper, which provides the ability to
deﬁne model objects following the Active Record
design pattern, and using a DSL syntax.”

Plan of Action

• SQLAlchemy Overview
• Concepts
• Example
• Elixir Overview
• Concepts
• Example
• Questions and Advanced Topics

SQLAlchemy Concepts

• Engine
• MetaData
• Table
• Mapper
• Session
• Query

SQLAlchemy Concepts: Engine

• Connection handling and pooling.
• Deﬁnes the “dialect” for database-speciﬁc translation.
• PostgreSQL, MySQL, Oracle, MS-SQL, Firebird, Informix, SQLite.

# create an engine
engine = create_engine('postgres://user:pass@host/database')

# perform a query
connection = engine.connect()
result = connection.execute('select * from users')
for row in result:
print row['username'], row['email']

# close the connection
connection.close()

SQLAlchemy Concepts: MetaData

• A collection of “schema items” like tables and indexes.
• Can be “bound” to an engine.

# create a metadata and bind it to an already-created engine
metadata = MetaData()
metadata.bind = engine

# issue CREATE statements for objects within this metadata
metadata.create_all()

SQLAlchemy Concepts: Table

• A table in your SQL database.
• Associated with a MetaData.
• Can perform SQL operations against tables.
• Can be “reﬂected” from existing schemas (autoload=True).

users_table = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('username', Unicode(16), nullable=False),
Column('email', Unicode(60)),
Column('password', Unicode(20), nullable=False)
)


# insert a row into the users table
users_table.insert().execute(
username='cleverdevil',
email='jonathan@cleverdevil.org',
password='s3cr3tw0rd'
)


# select all rows from the users table
rows = users_table.select().execute()
for row in rows:
print row['username'], row['email']

# select certain rows from the users table
clever_not_devil_rows = users_table.select(
and_(users_table.c.username.like('%clever%'),
not_(users_table.c.username.like('%devil%')))
).execute()

SQLAlchemy Concepts: Mapper

• Part of the Object Relational Mapper (ORM).
• Associates a table (or other “selectable” schema object) with a class.
• Deﬁnes relationships between objects.

class User(object):
def __init__(self, username, email, password):
self.username = username
self.email = email
self.password = password

mapper(User, users_table)

SQLAlchemy Concepts: Session

• Part of the Object Relational Mapper (ORM).
• Represents a “Unit of Work” which is a collection of objects to be persisted.
• Objects are “saved” to a session, then “ﬂushed” to the database.

# create a session and a user object
session = create_session()

user = User()
user.username = 'cleverdevil'
user.email = 'jonathan@cleverdevil.org'
user.password = 's3cr3tw0rd'

# save the object to the session and flush it
session.save(user)
session.flush()

SQLAlchemy Concepts: Query

• Query objects used to fetch mapped objects from the database.
• Often created through a session.
• Generatively build select statements.

# create a query object for User class
query = session.query(User)


# get all User objects from the database
for user in query.all():
print user.name, user.email

# get a single User object from the database
cleverdevil = query.get_by(username='cleverdevil')


# get the top 5 users with "clever" but not
# "devil" in their username in alphabetical order
users = query.filter(
User.c.username.like('%clever%')
).filter(
~User.c.username.like('%devil%')
).order_by(User.c.username).limit(5)

SQLAlchemy Example

• Weblog Software
• People write Articles
• Plan: Tables, Classes, Mappers

SQLAlchemy Example: Tables

metadata = MetaData()

person_table = Table('person', metadata,
Column('name', Unicode),
Column('email', Unicode),
Column('password', Unicode)
)

article_table = Table('article', metadata,
Column('title', Unicode),
Column('description', Unicode),
Column('content', Unicode),
Column('author_id', Integer, ForeignKey(person_table.c.id))
)

SQLAlchemy Example: Objects

class Person(object):
def __init__(self, name=None, email=None, password=None):
self.name = name
self.email = email
self.password = password

class Article(object):
def __init__(self, title=None, description=None,
content=None, author=None):
self.title = title
self.description = description
self.content = content
self.author = author

SQLAlchemy Example: Mappers

mapper(Person, person_table, properties={
'articles' : relation(Article)
})

mapper(Article, article_table, properties={
'author' : relation(Person)
})

SQLAlchemy Example: Demonstration

# bind our metadata to an in-memory # create a session and save the post, which
# sqlite database, and create tables # will also save the dependant Person object
metadata.bind = 'sqlite:///' session = create_session()
metadata.create_all() session.save(blog_post)
session.flush()
# create a Person instance
jonathan = Person( # create another session and use it to query
name='Jonathan' # for our just created instance
email='jonathan@cleverdevil.org' session = create_session()
password='s3cr3t' query = session.query(Person)
) jonathan = query.get_by(name='Jonathan')

create an Article instance # walk through the object, printing out some
blog_post = Article( # properties, including related objects
title='Some Blog Post', print jonathan.name
description='Some description', print jonathan.articles[0].title
content=Some content, print jonathan.articles[0].author.name
author=jonathan
)

SQLAlchemy: In Summary

• We have just scratched the surface: SQLAlchemy is amazingly powerful.
• Polymorphic inheritance.
• Complex relationships.
• Mapping to arbitrary selectables.
• Useful extensions.
• Much, much more!
• Excellent SQL toolkit and connection handling.
• Extremely powerful optional ORM.

SQLAlchemy: Mike Bayer and Chuck Norris

Mike Bayer is a robot built by a higher order
of beings who think in relational algebra,
and can bend space-time to give him 36-48
hours of productive time in a single day.

He doesn’t sleep, he just codes with his
eyes closed.

Elixir: Design Patterns

• SQLAlchemy uses the “Data Mapper” design pattern
• Classes and tables aren’t one-to-one.
• Classes can be mapped to arbitrary selects.

• Elixir uses the “Active Record” design pattern.
• Classes and tables are one-to-one.
• Classes, tables, and mappers can be deﬁned in one step.
• Unlike some other “Active Record” ORMs, compound primary keys are
supported!

Elixir Concepts: Entity

• Deﬁnes your class, table, and mapper in a single step.
• Uses an easy-to-read domain-speciﬁc language (DSL).

class User(Entity):
has_field('username', Unicode(16))
has_field('email', Unicode(60))
has_field('password', Unicode(20))

Elixir Example

• Weblog Software (again) – People write Articles
• Plan: deﬁne entities.

class Person(Entity):
has_field('name', Unicode)
has_field('email', Unicode)
has_field('password', Unicode)

has_many('articles', of_kind='Article', inverse='author')

class Article(Entity):
has_field('title', Unicode)
has_field('description', Unicode)
has_field('content', Unicode)

belongs_to('author', of_kind='Person', inverse='articles')

Elixir Example: Demonstration

# bind our metadata to an in-memory # elixir automatically creates a threadlocal
# sqlite database, and create tables # session for you using “SessionContext”
metadata.bind = 'sqlite:///' objectstore.flush(); objectstore.clear()
metadata.create_all()
# elixir provides a convenience function for
# create a Person instance # getting query objects
jonathan = Person( query = Person.query()
name='Jonathan' jonathan = query.get_by(name='Jonathan')
email='jonathan@cleverdevil.org'
password='s3cr3t' # walk through the object, printing out some
) # properties, including related objects
print jonathan.name
create an Article instance print jonathan.articles[0].title
blog_post = Article( print jonathan.articles[0].author.name
title='Some Blog Post',
description='Some description',
content=Some content,
author=jonathan
)

Elixir vs. SQLAlchemy

• Lines of Code for Weblog Data Model
• SQLAlchemy weighs in at 38 lines.
• Elixir comes in at 16 lines.
• Thats 58% less code.
• Sometimes you don’t need all the power of SQLAlchemy
• But Elixir is pretty powerful in its own right....

More Elixir: Events

• Watch for changes on your entities
• Decorators for before and after insert, update, and delete.

from elixir import *
from elixir.events import after_update


@after_update
def notify_author(self):
print 'Notifying', self.author.email
print ' -> article:', self.title, 'was updated'

More Elixir: Encryption

• Automatically encrypts and decrypts ﬁelds using Blowﬁsh.

from elixir.ext.encrypted import acts_as_encrypted

class Person(Entity):
has_field('name', Unicode)
has_field('email', Unicode)
has_field('password', Unicode)

has_many('articles', of_kind='Article', inverse='author')
acts_as_encrypted(for_fields=['password'], with_secret='abc123')

More Elixir: Versioning

• Automatically tracks versions of your entities.
• Reverting, version comparison, timestamps, version numbers, after_revert event.

from elixir.ext.versioned import *

acts_as_versioned()

@after_revert
def notify_author(self):
print 'Notifying', self.author.email
print ' -> article:', self.title, 'was reverted'

More Elixir: Versioning

# loop through previous versions of an article, showing differences
article = Article.query().get_by(title='Some Post')
print 'Current version:', article.version, article.timestamp
for version in article.versions:
print ' * Old version:', version.version, version.timestamp
print ' - differences ->', article.compare_with(version)

# get the version of this article as of a date and time
version = article.get_as_of(timestamp)

# revert to previous version
article.revert()

# revert to specific version number
article.revert_to(2)

More Elixir: Associables

• Create common association patterns easily
• Examples: addressable, taggable.

from elixir.ext.associable import * # loop through the tags on an article
for tag in article.tags:
class Tag(Entity): print 'Article', article.title
has_field('name', Unicode) print ' -> tagged as', tag.name

acts_as_taggable = associable(Tag) # fetch articles that are tagged with a
# certain tag
class Article(Entity): python_posts = Article.select_by_tags(
has_field('title', Unicode) name='python'
has_field('description', Unicode) )

acts_as_taggable('tags')

More Elixir: Lower Level Operations

• Elixir is built on SQLAlchemy.
• You can always drop down to table objects.

results = select(
[Person.c.email, Article.c.title],
and_(
Person.c.name == 'Jonathan',
Person.c.id==Article.c.author_id
)
).execute()

person_rows = Person.table.select().execute()

Elixir: Summary

• Simpliﬁed layer on SQLAlchemy
• Active Record instead of Data Mapper
• Lower-level operations always available

• Cross-cutting functionality through Elixir Statements
• Associable
• Encryption
• Versioning
• Write your own!

Questions

can_i_do_it_with_sqlalchemy = lambda it: ‘Yep’

Elixir http://elixir.ematia.de
SQLAlchemy http://sqlalchemy.org
Jonathan LaCour http://cleverdevil.org

Slides python elixir

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