Use a foreign key mapping to get data from the other table using Python and SQLAlchemy.

Posted by Az on Stack Overflow See other posts from Stack Overflow or by Az
Published on 2010-05-27T04:25:52Z Indexed on 2010/05/27 4:31 UTC
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Hmm, the title was harder to formulate than I thought.

Basically, I've got these simple classes mapped to tables, using SQLAlchemy. I know they're missing a few items but those aren't essential for highlighting the problem.

class Customer(object):
    def __init__(self, uid, name, email):
        self.uid = uid
        self.name = name
        self.email = email

    def __repr__(self):
        return str(self)

    def __str__(self):
        return "Cust: %s, Name: %s (Email: %s)" %(self.uid, self.name, self.email)  

The above is basically a simple customer with an id, name and an email address.

class Order(object):
    def __init__(self, item_id, item_name, customer):
        self.item_id = item_id
        self.item_name = item_name
        self.customer = None

    def __repr__(self):
        return str(self)

    def __str__(self):
        return "Item ID %s: %s, has been ordered by customer no. %s" %(self.item_id, self.item_name, self.customer)  

This is the Orders class that just holds the order information: an id, a name and a reference to a customer. It's initialised to None to indicate that this item doesn't have a customer yet. The code's job will assign the item a customer.

The following code maps these classes to respective database tables.

# SQLAlchemy database transmutation
engine = create_engine('sqlite:///:memory:', echo=False)
metadata = MetaData()

customers_table = Table('customers', metadata,
    Column('uid', Integer, primary_key=True),
    Column('name', String),
    Column('email', String)
)


orders_table = Table('orders', metadata,
    Column('item_id', Integer, primary_key=True),
    Column('item_name', String),
    Column('customer', Integer, ForeignKey('customers.uid'))
)

metadata.create_all(engine)
mapper(Customer, customers_table)
mapper(Orders, orders_table)  

Now if I do something like:

for order in session.query(Order):
    print order  

I can get a list of orders in this form:

Item ID 1001: MX4000 Laser Mouse, has been ordered by customer no. 12  

What I want to do is find out customer 12's name and email address (which is why I used the ForeignKey into the Customer table). How would I go about it?

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