How to map one class against multiple tables with SQLAlchemy?

Question

Lets say that I have a database structure with three tables that look like this:

items
 - item_id
 - item_handle

attributes
 - attribute_id
 - attribute_name

item_attributes
 - item_attribute_id
 - item_id
 - attribute_id
 - attribute_value

I would like to be able to do this in SQLAlchemy:

item = Item('item1')
item.foo = 'bar'

session.add(item)
session.commit()

item1 = session.query(Item).filter_by(handle='item1').one()
print item1.foo # => 'bar'

I'm new to SQLAlchemy and I found this in the documentation (http://www.sqlalchemy.org/docs/05/mappers.html#mapping-a-class-against-multiple-tables):

j = join(items, item_attributes, items.c.item_id == item_attributes.c.item_id). \
    join(attributes, item_attributes.c.attribute_id == attributes.c.attribute_id)

mapper(Item, j, properties={
    'item_id': [items.c.item_id, item_attributes.c.item_id],
    'attribute_id': [item_attributes.c.attribute_id, attributes.c.attribute_id],
})

It only adds item_id and attribute_id to Item and its not possible to add attributes to Item object.

Is what I'm trying to achieve possible with SQLAlchemy? Is there a better way to structure the database to get the same behaviour of "dynamic columns"?

Answer 1

This is called the entity-attribute-value pattern. There is an example about this under the SQLAlchemy examples directory: vertical/.

If you are using PostgreSQL, then there is also the hstore contrib module that can store a string to string mapping. If you are interested then I have some code for a custom type that makes it possible to use that to store extended attributes via SQLAlchemy.

Another option to store custom attributes is to serialize them to a text field. In that case you will lose the ability to filter by attributes.