SQLAlchemy

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SQLAlchemy
Original author(s) Michael Bayer[1]
Initial release February, 2006[2]
Stable release 0.9.6 / 23 June 2014; 31 days ago (2014-06-23)
Development status Active
Written in Python
Operating system Cross-platform
Type Object-relational mapping
License MIT License
Website www.sqlalchemy.org
Mike Bayer talking about SQLAlchemy at PyCon 2012

SQLAlchemy is an open source SQL toolkit and object-relational mapper (ORM) for the Python programming language released under the MIT License.

SQLAlchemy provides "a full suite of well known enterprise-level persistence patterns, designed for efficient and high-performing database access, adapted into a simple and Pythonic domain language". SQLAlchemy's philosophy is that SQL databases behave less and less like object collections the more size and performance start to matter, while object collections behave less and less like tables and rows the more abstraction starts to matter. For this reason it has adopted the data mapper pattern (like Hibernate for Java) rather than the active record pattern used by a number of other object-relational mappers.[3] However, optional plugins such as Elixir and declarative allow users to develop using declarative syntax.

SQLAlchemy was first released in February, 2006 and has quickly become one of the most widely used object-relational mapping tools alongside Django's ORM in the Python community.

Example[edit]

The following example represents an n-to-1 relationship between movies and their directors. It is shown how user-defined Python classes create according database tables, how instances with relationships are created from either side of the relationship, and finally how the data can be queried—illustrating automatically-generated SQL queries for both lazy and eager loading.

Schema definition[edit]

Creating two Python classes and according database tables in the DBMS:

from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relation, sessionmaker
 
Base = declarative_base()
 
class Movie(Base):
    __tablename__ = 'movies'
 
    id = Column(Integer, primary_key=True)
    title = Column(String(255), nullable=False)
    year = Column(Integer)
    directed_by = Column(Integer, ForeignKey('directors.id'))
 
    director = relation("Director", backref='movies', lazy=False)
 
    def __init__(self, title=None, year=None):
        self.title = title
        self.year = year
 
    def __repr__(self):
        return "Movie(%r, %r, %r)" % (self.title, self.year, self.director)
 
class Director(Base):
    __tablename__ = 'directors'
 
    id = Column(Integer, primary_key=True)
    name = Column(String(50), nullable=False, unique=True)
 
    def __init__(self, name=None):
        self.name = name
 
    def __repr__(self):
        return "Director(%r)" % (self.name)
 
engine = create_engine('dbms://user:pwd@host/dbname')
Base.metadata.create_all(engine)

Data insertion[edit]

Inserting movies and their directors can be achieved via both entities:

Session = sessionmaker(bind=engine)
session = Session()
 
m1 = Movie("Robocop", 1987)
m1.director = Director("Paul Verhoeven")
 
d2 = Director("George Lucas")
d2.movies = [Movie("Star Wars", 1977), Movie("THX 1138", 1971)]
 
try:
    session.add(m1)
    session.add(d2)
    session.commit()
except:
    session.rollback()

Querying[edit]

alldata = session.query(Movie).all()
for somedata in alldata:
    print somedata

SQLAlchemy issues the following query to the DBMS (omitting aliases):

SELECT movies.id, movies.title, movies.YEAR, movies.directed_by, directors.id, directors.name
FROM movies LEFT OUTER JOIN directors ON directors.id = movies.directed_by

The output:

Movie('Robocop', 1987L, Director('Paul Verhoeven'))
Movie('Star Wars', 1977L, Director('George Lucas'))
Movie('THX 1138', 1971L, Director('George Lucas'))

Setting lazy=True (default) instead, SQLAlchemy would first issue a query to get the list of movies and only when needed (lazy) for each director a query to get the name of the according director:

SELECT movies.id, movies.title, movies.YEAR, movies.directed_by
FROM movies
 
SELECT directors.id, directors.name
FROM directors
WHERE directors.id = %s

See also[edit]

References[edit]

Notes

External links[edit]