# SqlAlchemy Basics

## Engines and Sessions

The docs for engines and sessions.

You can create a basic sqlite engine.

    from sqlalchemy import create_engine
engine = create_engine('sqlite:///:memory:')


Given:

• user: scott
• host: localhost
• database: test

The equivalent URI would be:

    engine = create_engine("postgresql://scott:tiger@localhost/test")
engine = create_engine("mysql://scott:tiger@hostname/test",
encoding='latin1', echo=True)


To interact with the engine you need to use a session. You can create a session using a sessionmaker.

    from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine)

some_session = Session()


## Models

The Docs

An example of a basic model...

    from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String

Base = declarative_base()

class User(Base):
__tablename__ = 'users'

id = Column(Integer, primary_key=True)
name = Column(String)
fullname = Column(String)

def __repr__(self):
return "<User(name='%s', fullname='%s', password='%s')>" % (


It's pretty easy to create the tables that correspond to your models.

    Base.metadata.create_all(engine)


Dropping all tables and data is just as simple.

    Base.metadata.drop_all(engine)


## Creating Data

The Docs Once you have a session and have defined some models, you can create instances of that model.

    ed_user = User(name='ed', fullname='Ed Jones', password='edspassword')


You can then stage the instance to your session (similar to how you stage changes before committing them with git).

    session.add(ed_user)


You can then commit everything in one hit.

    session.commit()


Or roll back, if you don't want to save your changes.

    session.rollback()


The Docs

## Many-to-Many Relationships

This can be a tricky topic, so make sure to read the docs!

Many to Many adds an association table between two classes. The association table is indicated by the secondary argument to relationship(). Usually, the Table uses the MetaData object associated with the declarative base class, so that the ForeignKey directives can locate the remote tables with which to link.

    from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

Column('left_id', Integer, ForeignKey('left.id')),
Column('right_id', Integer, ForeignKey('right.id')))


Now to link the two classes, you need to declare a relationship attribute. The secondary argument specifies the association table used. The back_populates argument tells SqlAlchemy which column to link with when it joins the two tables. It allows you to access the linked records as a list with something like Parent.children.

    class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship(
"Child",
secondary=association_table,
back_populates="parents")

class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
parents = relationship(
"Parent",
secondary=association_table,
back_populates="children")


Alternatively, you can use the backref argument when specifying the relationship for one class, this creates a virtual column in the corresponding class that links back to the first one.

    class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship("Child",
secondary=association_table,
backref="parents")

class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)