This article is a brief memo for how to use SQLAlchemy and DB migration using Alembic. The best way to learn them is to read their document directly though they are nice volume. I, therefore, pick up some useful and basic snipets from document.
SQLAlchemy is ORM(Object-relational mapping). You can map your Python code to DB. It means you do not need to write SQL directly for handling data. But ORM library does not provide what you expected. You might write SQL.
DB migration is a way of changing your database schema from one version into another. When you add, delete schema and so on, these action can be managed by DB migration tools.
I will write abour how to qury using SQLAlchemy later.
SQLAlchemy
How to connect to DBs http://docs.sqlalchemy.org/en/rel_1_0/core/engines.html#database-urls
from sqlalchemy import create_engine
MySQL
# default
engine = create_engine('mysql://scott:tiger@localhost/foo')
# mysql-python
engine = create_engine('mysql+mysqldb://scott:tiger@localhost/foo')
# MySQL-connector-python
engine = create_engine('mysql+mysqlconnector://scott:tiger@localhost/foo')
# OurSQL
engine = create_engine('mysql+oursql://scott:tiger@localhost/foo')
SQLite
# sqlite://<nohostname>/<path>
# where <path> is relative:
engine = create_engine('sqlite:///foo.db')
Postgres
# default
engine = create_engine('postgresql://scott:tiger@localhost/mydatabase')
# psycopg2
engine = create_engine('postgresql+psycopg2://scott:tiger@localhost/mydatabase')
# pg8000
engine = create_engine('postgresql+pg8000://scott:tiger@localhost/mydatabase')
Declare a Mapping
Declaretive is mapping python class to DB schema. This is done by inheriting declaretive base class
http://docs.sqlalchemy.org/en/rel_1_0/orm/extensions/declarative/index.html
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'some_table'
id = Column(Integer, primary_key=True)
name = Column(String(50))
Mixin class allows to declare primary key easy.
from sqlalchemy.ext.declarative import declared_attr
class DBMixin(object):
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
__table_args__ = {'mysql_engine': 'InnoDB'}
__mapper_args__= {'always_refresh': True}
id = Column(Integer, primary_key=True)
If DBMixin is inherited by a declarative class for Table definition, the above User example becomes simpler.
class User(DBmixin, Base):
name = Column(String(50))
created_at, update_at
http://docs.sqlalchemy.org/en/rel_1_0/core/defaults.html
default is invoked when INSERT query is executed if no other value is supplied.
onupadte is invovoked when UPDATE query is executed and no values is supplied.
These are call by sqlalchemy; therefore it should be callable object(function etc..).
import datetime
from sqlalchemy import Column, Integer, DateTime
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Test(Base):
__tablename__ = 'test'
id = Column(Integer, primary_key=True)
created_at = Column(DateTime, default=datetime.datetime.now)
updated_at = Column(DateTime, default=datetime.datetime.now, onpudate=datetime.datetime.now)
Constrants and Indexes http://docs.sqlalchemy.org/en/rel_1_0/core/constraints.html
Basically, these are defined by argument in Column
Primary Key
Column('id', Integer, primary_key=True)
Composite Primary Key
PrimaryKeyConstraint('id', 'version_id', name='mytable_pk')
Column('id', Integer, primary_key=True),
Column('version_id', Integer, primary_key=True),
Foregin Key
Column('user_id', Integer, ForeignKey("user.user_id"), nullable=False),
Composite Foregin Key
Add primary_key in different row. SQLAlchemy atuomatically handles it.
Unique
Column('col1', Integer, unique=True)
Composite Unique
This is more explicit way.
from sqlalchemy import UniqueConstraint
UniqueConstraint('col2', 'col3', name='uix_1')
Index
Column('col1', Integer, index=True),
Column('col2', Integer, index=True, unique=True),
Composite Index
# place an index on col1, col2
Index('idx_col12', 'col1', 'col2'),
# place a unique index on col3, col4
Index('idx_col34', 'col3', 'col4', unique=True)
Nullable
Column('pref_name', String(40), nullable=False),
data type
http://docs.sqlalchemy.org/en/rel_0_9/core/type_basics.html#generic-types
There are some each DB specific data types.
Relation
one to one
An important thing,here is to specify uselist=False .
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
child = relationship("Child", uselist=False, backref="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
one to many
class Department(Base):
__tablename__ = 'department'
id = Column(Integer, primary_key=True)
name = Column(String)
class Employee(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
name = Column(String)
# employees is added in Department as an attribute for birateral relationship
departments = relationship(
Department,
backref='employees'
)
many to many
class Department(Base):
__tablename__ = 'department'
id = Column(Integer, primary_key=True)
name = Column(String)
# employees can be hanlded as python list.
# the string of name of class is okay for first arguemnt of relationship
employees = relationship(
'Employee',
secondary='department_employee'
)
class Employee(Base):
__tablename__ = 'employee'
id = Column(Integer, primary_key=True)
name = Column(String)
# employees can be hanlded as python list.
departments = relationship(
Department,
secondary='department_employee'
)
class DepartmentEmployee(Base):
__tablename__ = 'department_employee'
department_id = Column(Integer, ForeignKey('department.id'))
employee_id = Column(Integer, ForeignKey('employee.id'))
Index('deparment_employee_idx, 'deparment_id', 'employee_id', unique=True)
Migration
Use alembic
SQLAlchemy-migrate is not under active development. It seems that it stopped to adopt new SQLAlchemy(0.8 of SQLAlchemy is the latest support). While, alembic is maintained by SQLAlemy author. Bug fix is done really quick. Alembic is develped in Bitbucket. Github is a mirror of BitBucket.
ref: https://alembic.readthedocs.org/en/latest/
Install Alembic
pip install alembic
Initiation
Directory hierarchy
$ alembic init alembic
app
├── alembic
│ ├── README
│ ├── env.py
│ ├── script.py.mako
│ └── versions
├── model.py
Auto migrate template detection Cool feature of alembic is detect changes automatically compared database and revison files. NOTE: auto migration does no detect all changes.
Set up
Modify env.py
# imprt model.py
import os
import sys
MODEL_PATH = os.path.join(os.path.abspath(os.path.dirname(__file__)), "..")
sys.path.append(MODEL_PATH)
import model
# edit this line and pass metadata
target_metadata = model.Base.metadata
Can
Cannot
migrate
https://alembic.readthedocs.org/en/latest/tutorial.html#creating-an-environment
Create auto migrate template
$ alembic revision --autogenerate -m "initial"
Execute migration
head means the most recent change. It will migrate untill the most recent one.
$ alembic upgrade head
Show migration
$ alembic history --verbose
output
Rev: 340434aac9a (head)
Parent: 14db12dc041
Path: /Users/ken/proto/python/sqlalchemy/test/alembic/versions/340434aac9a_add_address.py
add address
Revision ID: 340434aac9a
Revises: 14db12dc041
Create Date: 2015-10-25 23:40:55.398984
Rev: 14db12dc041
Parent: <base>
Path: /Users/ken/proto/python/sqlalchemy/test/alembic/versions/14db12dc041_initial.py
initial
Revision ID: 14db12dc041
Revises:
Create Date: 2015-10-25 23:27:39.622146
License
SQLAlchemy
Alembic
Ref http://docs.sqlalchemy.org/en/rel_1_0/contents.html
http://pythoncentral.io/series/python-sqlalchemy-database-tutorial/
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