SQLAchemy is a powerful ORM for python, and is quite common in the Flask and Pyramid communities. It is installable via a simple:
pip install sqlalchemy
I’m going to use an in memory database just to play with, and to connect it we use create_engine supplied by SQLAlchemy
>>> from sqlalchemy import create_engine >>> engine = create_engine('sqlite:///:memory:', echo=True)
Next I’m going to use Declarative, which is a set of directives that creates classes and define tables relative to that class. We’ll use a base class which has a listing of all the classes and tables and is called a declarative base. SQLAlchemy provides one of these ready for use.
>>> from sqlalchemy.ext.declarative import declarative_base >>> Base = declarative_base()
Now lets look at the model we’re going to play with. We’ll be pulling in some class from SQLAlchemy that represent the components and datatypes of our tables.
>>> from sqlalchemy import Column, Integer, String, Float >>> class User(Base): ... __tablename__ = 'users' ... id = Column(Integer, primary_key=True) ... name = Column(String) ... fullname = Column(String) ... balance = Column(Float) ... group = Column(String) ... def __init__(self, name, fullname, balance, group): ... self.name = name ... self.fullname = fullname ... self.balance = balance ... self.group = group
Now let’s have the declarative base use the engine to build our table.
Okay now that we have a Model and a table and they are connected, we can add some data.
>>> user1 = User('Bob', 'Big Bob', 1000000.00, 'Mob') >>> user2 = User('Linda', 'Linda Lu', 100.50, 'Diner') >>> user3 = User('Lil Bob', 'Bobby Jr', 100500.00, 'Mob') >>> user4 = User('Rachael', 'Rachael Rach', 125.50, 'Personal')
Next, we want to save this data and to do that safely, we’ll need a session. We import the sessionmaker and bind it to our engine. After that, we create an instance of the session that we can use.
>>> from sqlalchemy.orm import sessionmaker >>> Session = sessionmaker(bind=engine) >>> db = Session()
Now let’s add our users to the sessions and commit/save them to the database.
>>> db.add(user1) >>> db.add(user2) >>> db.add(user3) >>> db.add(user4) >>> db.commit()
Okay finally, we can do some querying and play with grouping. To get all the user objects we could just do the following:
We can iterate though them and display useful data by doing.
>>> for user in db.query(User).all(): ... print user.name, user.balance Bob 1000000.0 Linda 100.5 Lil Bob 100500.0 Rachael 125.5
But what if we wanted a total of the balance by Group the user is in? To do that we need some more pieces from SQLAlchemy. Namely, we want to get a func class that will give us access to math based functions and label to let us label the returned data.
>>> from sqlalchemy import func >>> from sqlalchemy.sql import label
Okay, now we can put this all together to get a list of groups, how many people are in the group, and what the groups aggregated balance is.
>>> db.query(User.group, ... label('members', func.count(User.id)), ... label('total_balance', func.sum(User.balance))).group_by(User.group).all()
Remember that all fields you want to return in this type of query must use an aggregation function like count or be included in the group_by fields. The above query returns the results below.
[(u'Diner', 1, 100.5), (u'Mob', 2, 1100500.0), (u'Personal', 1, 125.5)]
And of course, we can also save them to a results list and print them out nicer. Notice how we can use the label names we access earlier to access that data.
>>> results = db.query(User.group, ... label('members', func.count(User.id)), ... label('total_balance', func.sum(User.balance))).group_by(User.group).all() >>> for result in results: ... print "%s has %i members with a balance of %d" % (result.group, result.members, result.total_balance) ... Diner has 1 members with a balance of 100 Mob has 2 members with a balance of 1100500 Personal has 1 members with a balance of 125
The full code from the example is available at this Gist.
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