SQLAlchemy

SQLAlchemy is an open-source Python library that provides an SQL toolkit and an object–relational mapper for database interactions. It allows developers to work with databases using Python objects, enabling efficient and flexible database access.

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Description

SQLAlchemy offers tools for database schema generation, querying, and object-relational mapping. Key features include:

A comprehensive embedded domain-specific language for SQL in Python called "SQLAlchemy Core" that provides means to construct and execute SQL queries.
A powerful ORM that allows the mapping of Python classes to database tables.
Support for database schema migrations.
Compatibility with multiple database backends.
Tools for database connection pooling and transaction management.

History

SQLAlchemy was first released in February 2006. It has evolved to include a wide range of features for database interaction and has gained popularity among Python developers. Notable versions include:

Version 0.1 : Initial release.
Version 1.0 : Major enhancements in ORM and SQL expression language.
Version 1.4 : Introduction of a new ORM API.

Example

The following example represents an n-to-1 relationship between movies and their directors. It is shown how user-defined Python classes create corresponding 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

Creating two Python classes and corresponding 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:
__tablename__ = "movies"
id = Column
title = Column
year = Column
directed_by = Column
director = relation
def __init__:
self.title = title
self.year = year
def __repr__:
return f"Movie"
class Director:
__tablename__ = "directors"
id = Column
name = Column
def __init__:
self.name = name
def __repr__:
return f"Director"
engine = create_engine
Base.metadata.create_all

Data insertion

One can insert a director-movie relationship via either entity:

Session = sessionmaker
session = Session
m1 = Movie
m1.director = Director
d2 = Director
d2.movies =
try:
session.add
session.add
session.commit
except:
session.rollback

Querying

alldata = session.query.all
for somedata in alldata:
print

SQLAlchemy issues the following query to the DBMS :

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
Movie
Movie

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

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